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Investigation on the Luminescence Properties of InMO4 (M = V5+, Nb5+, Ta5+) Crystals Doped with Tb3+ or Yb3+ Rare Earth Ions

  • Pablo Botella*
    Pablo Botella
    Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-97187 Luleå, Sweden
    *E-mail: [email protected]
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    Orcidhttp://orcid.org/0000-0001-6930-8415
  • , 
  • Francesco Enrichi
    Francesco Enrichi
    Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-97187 Luleå, Sweden
    Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, via Torino 155, 30172 Venezia, Italy
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  • , 
  • Alberto Vomiero
    Alberto Vomiero
    Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-97187 Luleå, Sweden
    Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, via Torino 155, 30172 Venezia, Italy
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    Orcidhttp://orcid.org/0000-0003-2935-1165
  • , 
  • Juan E. Muñoz-Santiuste
    Juan E. Muñoz-Santiuste
    Departamento de Física, MALTA Consolider Team, Escuela Politécnica Superior, Universidad Carlos III de Madrid, Avenida de la Universidad 30, E-28913 Leganés, Spain
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  • , 
  • Alka B. Garg
    Alka B. Garg
    High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India
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    Orcidhttp://orcid.org/0000-0003-4050-8469
  • , 
  • Ananthanarayanan Arvind
    Ananthanarayanan Arvind
    Process Development Division, Bhabha Atomic Research Centre, Mumbai 400085, India
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  • , 
  • Francisco J. Manjón
    Francisco J. Manjón
    Instituto de Diseño para la Fabricación y Producción Automatizada, MALTA Consolider Team, Universitat Politècnica de València, Camí de Vera s/n, 46022 València, Spain
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    Orcidhttp://orcid.org/0000-0002-3926-1705
  • , 
  • Alfredo Segura
    Alfredo Segura
    Departamento de Física Aplicada-ICMUV, Universidad de Valencia, MALTA Consolider Team, Edificio de Investigación, C. Dr. Moliner 50, 46100 Burjassot, Spain
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    Orcidhttp://orcid.org/0000-0002-9979-1302
  • , and 
  • Daniel Errandonea
    Daniel Errandonea
    Departamento de Física Aplicada-ICMUV, Universidad de Valencia, MALTA Consolider Team, Edificio de Investigación, C. Dr. Moliner 50, 46100 Burjassot, Spain
    More by Daniel Errandonea
    Orcidhttp://orcid.org/0000-0003-0189-4221
Cite this: ACS Omega 2020, 5, 5, 2148–2158
Publication Date (Web):January 30, 2020
https://doi.org/10.1021/acsomega.9b02862

Copyright © 2022 American Chemical Society. This publication is licensed under CC-BY.

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Abstract

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We explore the potential of Tb- and Yb-doped InVO4, InTaO4, and InNbO4 for applications as phosphors for light-emitting sources. Doping below 0.2% barely change the crystal structure and Raman spectrum but provide optical excitation and emission properties in the visible and near-infrared (NIR) spectral regions. From optical measurements, the energy of the first/second direct band gaps was determined to be 3.7/4.1 eV in InVO4, 4.7/5.3 in InNbO4, and 5.6/6.1 eV in InTaO4. In the last two cases, these band gaps are larger than the fundamental band gap (being indirect gap materials), while for InVO4, a direct band gap semiconductor, the fundamental band gap is at 3.7 eV. As a consequence, this material shows a strong self-activated photoluminescence centered at 2.2 eV. The other two materials have a weak self-activated signal at 2.2 and 2.9 eV. We provide an explanation for the origin of these signals taking into account the analysis of the polyhedral coordination around the pentavalent cations (V, Nb, and Ta). Finally, the characteristic green (5D4 → 7FJ) and NIR (2F5/2 → 2F7/2) emissions of Tb3+ and Yb3+ have been analyzed and explained.

Introduction

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Light-emitting diodes (LEDs) have attracted much attention in recent decades because of their properties of high brightness. (1) Lanthanide-doped oxides are suitable materials for these applications, particularly because of its robustness and flexibility for hosting different dopants. (1) Indium metal oxides with InMO4 (M = V5+, Nb5+, Ta5+) stoichiometry are included among them. These compounds are of interest not only because of its potential use as phosphors for LEDs (2,3) but also because of their ability to act as photocatalytic materials (4−6) and gas sensors. (7,8) All these applications are intimately related to the electronic band structure of the material. By modifying the electronic band structure, the optical and electronic properties of a given material can be tailored for specific applications. Several methods can be chosen to tune materials properties, such as high-pressure techniques, (9−14) ion irradiation, (15,16) and doping. (2,3,17) In particular, mechanical techniques modify material properties by deforming the lattice of the crystal. In contrast, chemical techniques, such as doping, modify the crystal structure very slightly for a doping below 1%, thus remaining the structure almost identical to the undoped sample. (2,3,17) However, the dopant, even in very small proportions, introduces localized electronic levels that have a significant impact on the electronic and optical properties. In this context, it has been demonstrated that doping InMO4 (M = V5+, Nb5+, Ta5+) materials improves their performance as photocatalysts. (18−22) Moreover, they have also shown to be good host materials for rare earth (RE) ions, being the luminescence properties useful for LEDs. (2,3,23,24)
The crystal structures of indium niobate (InNbO4) and indium tantalate (InTaO4) are isomorphic and belong to the monoclinic space group P2/c of the wolframite structure (see Figure 1, left). (9−11,25) The structure has two formulae per unit cell (Z = 2); Nb(Ta)5+ occupies the 2e sites, while In3+ occupies the 2f sites. Both Nb(Ta) and In cations feature a 6-fold coordination. In fact, both InO6 and Nb(Ta)O6 octahedral units are the building blocks of the structure because the crystal structure is constructed by edge- and corner-sharing of InO6 and Nb(Ta)O6 zigzag chains parallel to the c direction and layered in the a direction.

Figure 1

Figure 1. Crystal structure of the wolframite-type InNb(Ta)O4 host and coordination environments for Nb5+(Ta5+) and In3+ (left) and orthorhombic InVO4 host and coordination environments for V5+ and In3+ (right) (for the color code of the structure, the reader is referred to the digital version).

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Indium vanadate (InVO4) crystallizes in the orthorhombic space group Cmcm (Z = 4) with In3+ and V5+ atoms occupying 4a and 4c sites, respectively. (26,27) The structure is composed of InO6 octahedral units and VO4 tetrahedral units as building blocks. InO6 octahedra are edge-sharing along the c axis, forming chains that are connected through VO4 tetrahedral units (see Figure 1, right). The octahedral units are more regular than the ones in the wolframite structures of the other compounds, and VO4 units are not linked between them.
It has been recently shown that InVO4, InNbO4, and InTaO4 are wide band gap semiconductors. (10) These studies clarified the discrepancies reported in the literature about their band gap energy (Eg) and about their band gap nature. Many of the controversies were due to a wrong assignment of the fundamental absorption edge to the absorption of light by defects. (9,10) InVO4 was shown to be a direct band gap semiconductor along the Y → Y direction with an Eg of 3.62(5) eV, whereas InNbO4 and InTaO4 are indirect semiconductors along the Y → Γ-B direction with Eg values of 3.63(5) and 3.79(5) eV, respectively. In all the compounds, states at the bottom of the conduction band (CB) are dominated by V 3d, Nb 4d, or Ta 5d, and O 2p states dominate the upper part of the valence band (VB).
Extensive experimental and theoretical works on doping InVO4, InNbO4, and InTaO4 compounds by using nonmetal and metal elements have been reported. (17−22) However, regarding RE elements, mainly three of them (Eu3+, Tm3+, and Dy3+) have been used for doping such compounds, (2,3,23,24,28−31) and only one work has been reported on Tb-doped InTaO4. (31) Besides, it has been shown that InVO4, InNbO4, and InTaO4 can be self-activated phosphors depending on the synthesis process, which can lead to modification of the morphology, pH, and M/In molar ratio and consequently of the luminescence properties. (28,31−33)
Here, we report a detailed study on the luminescence and optical properties of InMO4 (M = V5+, Nb5+, Ta5+) compounds by comparing undoped materials and materials doped with Tb3+ or Yb3+ RE ions. The self-activated luminescence of the undoped samples and the influence of the host lattice in the characteristic green and near-infrared (NIR) region emission lines of Tb3+ and Yb3+, respectively, are also studied.
Table 1. Atomic % of the Dopants in InVO4, InNbO4, and InTaO4 Compounds
dopant sampleTb3+Yb3+
InVO40.190.17
InNbO40.090.08
InTaO40.070.15

Results and Discussion

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Structural and Vibrational Analysis

X-ray diffraction (XRD) patterns of all doped samples were collected in order to determine the crystal structure and purity and were compared to the XRD patterns of the undoped samples previously reported. (9,11,12)Table 2 and Figure 2 show the Rietveld refinement results and the XRD patterns (bars/columns under data represent the simulated XRD of undoped orthorhombic InVO4 and monoclinic InNbO4 and InTaO4), respectively. All diffraction peaks correspond to the orthorhombic InVO4 and wolframite InNbO4 and InTaO4 crystal structures. The host lattice was barely affected by the small concentration of the dopants used. Similar results have been also observed using other doping elements. (2,3,17) In these structures, In3+ has 6-fold octahedral coordination in all the samples, with an ionic radius of 0.8 Å. Considering the same valence and coordination, Tb3+ and Yb3+ have ionic radii of 0.923 and 0.868 Å, respectively. Therefore, RE ions can be assumed to occupy the In3+ sites in the InVO4, InNbO4, and InTaO4 host lattices. Results from XRD and RS are consistent with this hypothesis. Only some residual material from precursors have been observed in the case of InTaO4 doped with Yb3+ (see Table 2). Even though RE ions possess bigger ionic radii than In3+, it is observed that there is a small diminution of the lattice parameters leading to a reduction of the unit cell volume less than 1%, contrary to what would be expected (see Table 2). These variations of the lattice parameters could be ascribed to a distortion of the octahedral units when a foreign RE element is introduced into the crystal structure, which reduces the unit cell volume to accommodate the RE ions. As we will see later, these modifications will be reflected in the self-activated PL properties of the materials.

Figure 2

Figure 2. XRD patterns of InVO4, InNbO4, and InTaO4 doped samples with Tb3+ or Yb3+. Bars/columns data represent the standard ICSD charts of the undoped orthorhombic InVO4 (ICSD-237482) and the undoped monoclinic InNbO4 (ICSD-257869) and InTaO4 (ICSD-72569), respectively. The height of the bars is proportional to the theoretical intensity of the peaks. Tables indicating the index, positions, and intensities of all reflections are included in the Supporting Information.

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Table 2. Unit Cell Parameters and Goodness of the Rietveld Refinement for InVO4, InNbO4, and InTaO4 Compounds and the Corresponding Doped Samples with Tb3+ or Yb3+ from our XRD Experiments and the Contribution of the Residual Precursor Materials Found on the InTaO4:Yb Sample
 lattice parameters goodness of the fit
samplea (Å)b (Å)c (Å)β (°)ΔV (%)RpRwpRexp
InVO45.7588.5306.587  3.828.845.93
InVO4:Tb5.7478.5066.563 –0.87.0312.885.55
InVO4:Yb5.7448.5016.565 –0.96.6611.625.51
InNbO44.8365.7715.14491.13 5.89.545.8
InNbO4:Tb4.8305.7585.12991.19–0.64.938.14.32
InNbO4:Yb4.8325.7605.12991.17–0.64.86.475.86
InTaO44.8265.7755.15591.37 2.925.932.89
InTaO4:Tb4.8215.7675.14891.37–0.44.015.825.18
InTaO4:Yb4.8235.7675.14991.35–0.34.756.264.68
InTaO4:YbIn2O3Ta2O5InTaO4
contribution (%)7.46.286.4
Raman measurements also support XRD observations. As can be seen in Figure 3, the Raman signal of the doped samples is similar to that of the previously reported undoped samples. (9,11,12) Although the dopants slightly modify the unit cell, no appreciable shifts or broadenings of the peaks were observed on the results. This is due to a small local disorder introduced in the crystalline network. Notice that if the dopants were located at interstitial sites, that is, not substituting indium, more important changes (likely with the appearance of new Raman modes) could have been found in the Raman spectrum. Therefore, Raman measurements support the claim that the RE atoms substitute In. Regarding the small changes in Raman frequencies, this can be related to the small unit cell volume change associated with doping. The reduction of the unit cell volume due to dopants can be seen as the effect of an external applied pressure to the material equivalent to 0.64, 1.14, and 0.7 GPa for InVO4, InNbO4 and InTaO4, respectively. (9−12) These pressures would shift the Raman modes about 3–5 cm–1; however, we have observed all the shifts to be lower than 2 cm–1, that is, within the instrumental resolution.

Figure 3

Figure 3. RS spectra of InVO4, InNbO4, and InTaO4 compounds and the corresponding doped samples with Tb3+ or Yb3+.

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Thus, doping the sample with Tb3+ or Yb3+ does not modify appreciably the phonon frequencies of the compounds studied here. This also supports that RE are substituting the In3+ atoms, what excludes the possibility that they are placed in an interstitial position, which would, in principle, could give rise to local vibrational modes observable as RS peaks, such as those found in ZnO. (41)

Optical and Photoluminescence Properties

Optical reflectance and photoluminescence measurements of InVO4, InVO4:Tb, and InVO4:Yb samples are shown in Figure 4. The optical reflectance measurements for all samples are similar (see Figure 4, top), showing a broad asymmetric band feature from 2.8 to 4.6 eV with a maximum around 4.1 eV. For comparison, the calculated reflectance (R) has been also included. These data have been estimated using the calculated refractive index (n) by Mondal et al. (42) and using the Fresnel equation in the special case of normal incidence when the sample is immersed in air:
(1)

Figure 4

Figure 4. Optical reflectance (top) and PLE/PL spectra (bottom) of InVO4, InVO4:Tb, and InVO4:Yb (short dashed-dotted line corresponds to PLE data). The data were normalized for a better comparison of the emitted signals.

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The trend in the calculated reflectance is in good agreement with the observed spectra. However, due to the use of two different functionals, there is displacement of band gap energy from 3.13 eV with density functional theory calculations to 4.02 eV with tight binding calculations.
The broad asymmetric reflectance band can be well described by two Gaussian functions peaking at 3.7 and 4.1 eV. According to Mondal et al., (42) this energy region is due to the direct allowed interband transitions between the valence band and conduction band states caused by the charge transfer (CT) from O(p) to V(d) atoms inside the tetrahedral units. Thus, following previous results, we attribute these maximums to the two first direct optical allowed transition of InVO4 at the Y → Y and Γ → Γ points of the Brillouin zone (BZ). (10,27,42) The first direct transition value is in very good agreement with the energy gap value determined from our optical absorption measurements (Eg = 3.6 eV) (10) and consistent with the direct band gap nature. A reflectance maximum requires a strong optical absorption onset, which is the signature of an allowed direct transition. On the contrary, in indirect transitions, the optical transitions are very weak due to the need of phonon participation to conserve the momentum. Therefore, in direct gap semiconductors, the reflectance maximum and the fundamental absorption edge are expected at a similar energy. However, in indirect gap semiconductors, the first maximum of the reflectance corresponds to the first direct allowed transition, which is at a much higher energy than the indirect transition as it will be seen for the other two materials studied here.
The photoluminescence excitation (PLE) results are similar for all samples, showing a broad band starting at 3.4 eV and peaking around 4.1 eV (see Figure 4, bottom, short dashed-dotted line). For the PL signal, a very broad band similar for all the samples is seen peaking at 2.2 eV in the visible region (see Figure 4, bottom, solid line). InVO4 is a self-activated phosphor material due to the CT inside the vanadate group VO43–. The vanadate oxoanion in a distorted tetrahedral coordination different from the ideal Td symmetry where the transitions are spin-forbidden, presenting a self-activated luminescence due to the spin–orbit interaction that makes partly allowed the transitions. (43−45) In our case, the tetrahedral vanadate presents two different bond distances to the oxygen atoms, which makes the Td symmetry degraded to the subgroup C2v, giving rise to luminescence.
The PLE spectra show that the self-activated PL is excited by band to band transitions. These transitions were attributed to the direct transition from the ground state (1A1) due to the oxygen 2p localized states to the excited Teltow (T) levels of the 3d vanadate states (see Figure 5, top). Due to the 3d metal character, the first excited level gives rise to four states, which, following the same nomenclature as in literature, are called 1T1, 1T2, 3T1, and 3T2, with a proposed level ordering as 3T1 ≈ 3T2 < 1T1 < 1T2. (44)

Figure 5

Figure 5. Energy level diagram of InVO4 doped with Yb (top) and InNbO4 doped with Yb or Tb (ET stands for electron transfer. CR stands for cross-relaxation. Dashed arrows represent nonradiative processes, and solid arrows correspond to PL emission and excitation).

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Following previous analysis, (42−45) the PLE spectra can be deconvoluted using two Gaussian functions as it can be seen in Figure 4, bottom. The Gaussian functions are peaking at 3.7 and 4.1 eV, which agree well to the direct transitions observed in the reflectance measurements. These energies are attributed to the direct transition from 1A1 to 1T2 (Ex1) and 1T1 (Ex2), respectively (see Figure 5, top). The PL spectra were also deconvoluted using two Gaussian functions peaking at 2.1 and 2.4 eV. These energy levels correspond to the radiative decay of 3T2 → 1A1 (Em1) and 3T1 → 1A1 (Em2) inside the VO43– group.
In ref (46), a modest absorption in the visible light region above 2.5 eV was observed due to the existence of oxygen vacancies and defects in the InVO4 compound. However, they interpreted the observed luminescence as a consequence of these defects, forming a donor–acceptor pair that involves a deep donor state located at ∼0.7 eV below of the conduction band and an acceptor state that is located at ∼0.3 eV above the valence band. As it is discussed above and in ref (47), the luminescence is due to the CT inside the VO4 group and not to lattice defects or impurities as color centers. Instead, these defects or impurities play a role in the luminescence efficiency and exciton lifetime because they act as trap centers. Thus, the strong luminescence, a consequence of the distorted tetrahedral vanadate, could be affected by the presence of defects such as oxygen vacancies.
A small shift (<0.1 eV) can be observed in the PLE measurements, which could be attributed to the effect of the concentration and the distortion grade of the VO43– groups, which affects the lattice parameters and influences the energy level positions of the excited states 1T1 and 1T2. (44)
Only the self-activated PL emission from the host material was observed in the visible region, without any signal from the characteristic green emission lines of Tb3+, which lay in the same spectral region (see Figure 5, bottom). However, in a previous work, (8) it has been observed that there are characteristic Eu3+ lines even when InVO4 was doped with concentrations below to 2%. Different reasons have been suggested to the suppression of the emission lines of Tb3+ such as an inefficient energy transfer from the host material to the RE ions, a significant back-transfer rate, or a loss mechanism due to the Tb–V interaction via intervalence absorption. (48,49)
For the InVO4:Yb sample, the PL signal was detected in the NIR (see Figure 4, bottom). We can attribute the PL signal in the NIR to the transitions from 2F5/2 to 2F7/2 energy levels of the Yb3+ atoms as it can be seen in the energy level diagram (see Figure 5, top) calculated in the way previously indicated using the structural data. Although the energy level scheme of Yb3+ is very simple and contains two multiplets, the 2F7/2 ground state and the 2F5/2 excited state, the electronic energy level scheme resulting from the simulation cannot be assumed as very accurate. This fact is related with the strong interaction of Yb3+ions with the lattice vibration that usually gives rise to strong vibronic sidebands. In this case, the Yb3+ NIR luminescence consists of a broad band with small shoulders being difficult to identify the electronic transitions. Despite this uncertainty and to maintain the hypothesis that Yb3+ ions are actually substituting In3+ ions, we can attribute the PL signal in the NIR to the transitions from the 2F5/2 to 2F7/2 energy level of the Yb3+ atoms as it can be seen in the energy level diagram (see Figure 5, top). The proposed PL mechanism is as follows (see Figure 5, top): the UV photons are absorbed by the VO43– groups in the host matrix (fundamental absorption from O 2p in the valence band to the V 3d levels in the conduction band generating an electron–hole pair), which transfer part of the energy to the Yb3+ ions by a nonradiative mechanism (the remaining energy is self-emitted), in which the electron–hole is captured by Yb3+ ions. The excited Yb3+ ions come back to the ground state through a radiative transition (due to the thermal motion, all An (n = 1, 2, 3) levels are populated, and consequently, emissions from all of them are expected). (50)
Figure 6 shows the reflectance (top) and photoluminescence (bottom) spectra of InNbO4, InNbO4:Tb, and InNbO4:Yb samples. The calculated reflectance is also included, as estimated from the dielectric function calculated by Li et al. (51) and using eq 1 once the refractive index was calculated using the approximation , where ε1 is the real part of the dielectric function. Despite the displacement due to the different functional used, the calculated reflectance describes well the experimental trend.

Figure 6

Figure 6. Optical reflectance (top) and PLE/PL spectra (bottom) of InNbO4, InNbO4:Tb, and InNbO4:Yb (short dasheddotted line corresponds to PLE data). The data were normalized for a better comparison of the emitted signals.

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The optical reflectance spectra are similar for all the samples, showing two features around 4.7 and 5.3 eV. We identify these energies with the direct transitions from the VB to the CB at Γ → Γ and Z → Z points in the BZ, respectively. (10) These transitions yield different contributions to the dielectric function in different directions (anisotropic material), specifically for the xx component of the dielectric tensor, which show a shift to higher energies of the maximum with respect to yy and zz components. (51)
The PLE spectrum of undoped InNbO4, in the measured range, shows a PLE peak with a maximum centered at 4.6 eV. This energy matches the direct transition observed by optical reflectance, and it is due to the charge transfer from the filled oxygen p states to the empty niobate d states inside of the octahedral unit NbO43–. (23,28) However, due to the limitations of the PLE setup, we cannot observe the second maximum at a higher energy. From our previous work, (10) we know that InNbO4 is an indirect wide band gap semiconductor with the valence band maximum (VBM) at the Y point and the conduction band minimum (CBM) in a point between the Γ and B direction of the BZ. Our optical absorption measurements yield an indirect energy gap value of 3.6 eV, which is smaller than the values found by PLE and optical reflectance. This is due to the indirect gap nature of the material as discussed previously.
For undoped InNbO4, no fluorescence was found in the visible region (see Figure 6, bottom). Blasse et al. (28) reported similar results for undoped InNbO4 under UV radiation and only very weak blue emission at liquid nitrogen temperature. On the contrary, Feng et al. (52) found, in InNbO4 nanofibers and nanoparticles, a significant PL signal centered at 2.9 eV. This value well corresponds with the features found in the PLE results of the doped samples.
The PLE measurements of the doped samples exhibit several features apart from the fundamental absorption at 4.6 eV. In the case of InNbO4:Tb, the fundamental absorption also lays at the same level as the 7DJ of the Tb3+ ions where most probably overlap both absorption bands (see Figure 5, bottom). In general, the excitation of Tb3+ in the UV spectral region may have different origins. (53) It can be attributed to the charge transfer (CT) from the orbitals 2p of O2– to the 4f of Tb3+ (54,55) or to the spin-allowed transition between the 7F6 ground state and 7DJ (J = 1, 2, 3, 4, 5) multiplets (low spin 4f75d excited states of Tb3+). Similarly, the feature around 4 eV may be related to the energy level 9DJ of the Tb3+ ion. This band is due to the spin-forbidden transition between the 7F6 ground state and 9DJ (J = 3, 4, 5, 6) multiplets of the Tb3+ ion (high spin 4f75d excited states of Tb3+). (56,57) The feature at 2.9 eV, which also has been observed for the InNbO4:Yb sample, could be related to impurity states (i.e., oxygen vacancies) introduced by the doping process or to a transition in the NbO67– octahedral having an Oh symmetry that give rise to self-activated luminescence (i.e., the intrinsic luminescence of the material, which does not originate from doping) in that region. (52,58) These distorted octahedral units, with lower symmetry than that of the ideal octahedron, could be the responsible for the decrease in the lattice parameter observed by XRD measurements.
In the case of InNbO4:Yb, an additional band in the PLE spectrum is observed around 3.4 eV, which we called (E1) in the energy level diagram (see Figure 5, bottom). This band only was observed by monitoring the emission in the NIR region; thus, this level transfers all the energy to the Yb atom that in turn radiatively decays emitting the characteristic NIR lines of the Yb3+ ions. This E1 level is related to the CT band of the 2p orbital of oxygen to 4f orbital of Yb3+ ions, which would directly populate the excited 2F5/2 level (see Figure 5, bottom). (59,60)
The PL of the doped samples, besides the abovementioned feature around 2.9 eV, shows the characteristic green line emissions of Tb3+ atoms due to the 5D4 to 7FJ (J = 0, 1, 2, 3, 4, 5, 6) transitions (see Figure 5, bottom). In this case, the results obtained from the energy level simulation in highly satisfactory. Using the calculated 5D4 and 7FJ energy level positions, we can reproduce the position and width (due to the overlapping of the different transitions) of the observed emission bands. This fact strongly supports the hypothesis that Tb3+ are in substitutional configuration in the InNbO4 host matrix.
The 5D4 → 7F5 transition always has the largest probability. This fact comes from the largest values of the reduced matrix elements both the electric dipole and the magnetic dipole ones for this transition. (61) We do not observe the 5D3 → 7FJ luminescence, which is expected in the same spectral region around 2.9 eV. As commented previously, this feature is intrinsic of the material and not from the dopants. The 5D3 → 7FJ luminescence is expected to be obtained when Tb excitation takes place at higher energies because the large 5D3–5D4 energy separation (more than six lattice phonons) that makes the nonradiative 5D3–5D4 relaxation highly unlikely. (62) The absence of this luminescence indicates that the ET channel (Figure 5, bottom) from the host to the Tb ions does not take place at energies above 3.0 eV. Additionally, cross-relaxation mechanisms are often argued as the main 5D3–5D4 depopulation channel in some other oxides with a similar Tb concentration. (63,64)
For the sample doped with Yb3+, the characteristic NIR lines were observed at the same energy as in the InVO4 matrix but showing sharper emission peaks, suggesting a different interaction with the lattice and allowing a successful simulation from our crystal field calculation. Henderson and Imbusch showed that the electron–phonon coupling modifies the 4f electron wave-function description by introducing opposite-parity ones. (62) As a general result, the electron–phonon coupling takes part in many phenomena besides the vibronic sidebands, including shapes and widths of spectral lines and modification of the relaxation rates. (64) On this way, the different spectral shape obtained for the InNbO4 (and for InTaO4, see further in the text) samples must be strongly related with the different (richest) phonon structures obtained in the Raman spectra for these samples.
Similar results are expected in the case of InTaO4 as it has the same crystal structure as InNbO4. The reflectance (top) and photoluminescence (bottom) measurements of the InTaO4 undoped matrix and the doped samples are presented in Figure 7. As in InNbO4, the reflectance spectra exhibit two features around 5.6 and 6.1 eV, which are associated with the direct transitions at the same points in the BZ as in InNbO4, given the close similarity in the electronic band structure of both compounds. (9,10) The calculated reflectance was estimated in the same way as in the InNbO4, showing that these maximums are an intrinsic feature of the dielectric function.

Figure 7

Figure 7. Optical reflectance (top) and PLE/PL spectra (bottom) of InTaO4, InTaO4:Tb, and InTaO4:Yb (short dasheddotted line corresponds to PLE data). The data were normalized for a better comparison of the emitted signals.

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In the PLE measurement of the undoped sample, no maximum was observed due to the spectral range limitation. InTaO4 is a wider indirect band gap semiconductor (3.75 eV) by comparison to InNbO4. (9,10) Thus, the direct transitions are expected to be at a higher energy than those in InNbO4. This is because the distances of Ta–O in the TaO6 octahedra are smaller than those in the NbO6 octahedra, which makes the band gap wider.
In the PLE results of doped samples, basically, the same two features as in InNbO4:Tb can be seen for InTaO4:Tb. These features are due to Tb+3 ion absorption bands, and no contribution of the host in this case is possible due to the position of the fundamental absorption band at higher energies. However, an additional band can be seen around 3.9 eV, which gives rise to a broad band luminescence in the visible region. This band can be clearly seen for InTaO4:Yb (see Figures 6 and 7 for comparison), and most probably, in the case of InTaO4:Tb, this band overlaps with the self-absorption band of the Tb+3 ion at 3.9 eV. This signal was only observed for doped samples and again can be attributed to the defects introduced by the doping process or probably the formation of an additional distorted TaO67– octahedral site with symmetry C3v as suggested by Chukova et al. (58)
Brixner et al. (31) reports that a properly prepared InTaO4 exhibits self-activated PL around 3 eV, and Zeng et al. (33) observed the PL signal for InTaO4 nanofibers and nanoparticles with a broad peak centered at 2.7 eV. In our measurements, the weak PL signal around 2.9 eV for InTaO4 (see Figure 7, bottom) is due to the same process as discussed for InNbO4. A PL broad band from 1.7 to 2.6 eV is seen for InTaO4:Yb in the visible region. This can be due to a self-activated PL of the host material associated with defects and vacancies, created by doping, as it is also present in the PL signal from InTaO4:Tb. However, this emission is not observed in InNbO4 samples with Oh symmetry. Thus, most probably in the tantalate structure, there are octahedra with C3v symmetry that generate this additional excited and emission level in the matrix. (58) Similar as in InNbO4, the characteristic green emissions of Tb3+ atoms in the visible region are also observed in InTaO4. About the NIR emissions of Yb3+, it can be noticed that the emissions are stronger in InNbO4 than that in InTaO4 when normalized to the predominant emission at 2.24 eV.
Concerning time-resolved measurements, the decay curves and lifetimes of the characteristic emission lines of Tb3+ and Yb3+ and the self-activated InVO4 PL signal are shown in Figure 8 and Table 3, respectively. All the RE3+ emissions show double exponential decay, which is frequently observed when the excitation energy is transferred from the donor MO43– (M = V, Nb, Ta) to the activator ion Tb3+ or Yb3+ in this case. (65) In nonheavily doped samples, the characteristic behavior of the luminescence decays shows a typical initial fast decay (related with the transfer mechanism and faster as higher the concentration is) followed by a long living tail (mainly associated with the pure radiative decay).

Figure 8

Figure 8. Time-resolved PL decay in the visible and NIR spectral regions for each matrix. The excitation wavelength was 310 nm, and emission wavelengths were 550 nm for visible and 995 nm for NIR.

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Table 3. Lifetime Values of the Characteristic Emission Lines of Tb3+, Yb3+, and Self-Activated InVO4 Band
sampleemission (nm)B1τ1 (μs)B2τ2 (μs)τaverage (μs)
InVO4550    69
InVO4:Tb550    71
InVO4:Yb550    66
InVO4:Yb (NIR)995250025239823341997
InNbO4:Tb55079312344232210
InNbO4:Yb (NIR)9972439621659340249
InTaO4:Tb550226225755263202
InTaO4:Yb (NIR)99926321071325461241
The decay time of the self-activated PL of InVO4 shows similar values even when it is doped by RE3+ ions, indicating that the decay time of the self-activated emission of (VO4)3– is not affected by doping. This behavior has been previously observed for Sr3La(VO4)3 when changing the doping concentration of Eu3+ (no variation of the decay time was observed), and the values agree with the values reported in the literature for the self-activated emission of the VO4 tetrahedral units embedded in different structures. (43,66) This implies that the electron transfer (ET) between (VO4)3– and RE3+ ions is not owing to the cross-relaxation between 3T1, 3T2–1A1 of (VO4)3– and 5D4–7FJ of Tb3+, and the 2F5/2–2F7/2 of Yb3+. (43)
The characteristic lifetime emission of the Yb3+ in the NIR region (997 nm) is, in general, long-lived (around 1 ms), (67) which is consistent with our measured decay time for Yb3+ ion in the InVO4 host. However, when Yb3+ is hosted in InNbO4 or InTaO4, it shows similar values (0.2 ms) in both compounds due to the same environment of the RE ions in both isostructural compounds but with an unusual short decay time. Such a short lifetime could be attributed to the specific local environment or to a fast recombination such as defect states in these nanostructured materials. The short decay time could be a good feature for white LED application because it avoids saturation at a high excitation. (68) The lifetime of the characteristic emissions of Tb3+ in the visible region (550 nm) shows similar values in both InNbO4 and InTaO4 and is consistent with other compounds (69) due to the same environment of the RE as we commented previously for the case of Yb3+ ions. An interesting fact to explore in the future is the influence of the difference of maximum phonon energies in lifetime differences. (9,11,12)
Among the possible applications, these materials may be used as phosphors both in the visible and NIR spectral regions. In the visible region, an important parameter for the quality of the emitted color is given by the CIE chromatic coordinates. The calculated CIE (x, y) coordinates of InVO4, InNbO4:Tb, and InTaO4:Tb are shown in Figure 9, while Table 4 lists the CIE coordinates values, correlation color temperature (CCT), color rendering indices (CRI), and the color emitted from each sample. In InVO4, as it is a self-activated phosphor, they are only estimated for the undoped sample. InVO4, InNbO4:Tb, and InTaO4:Tb present yellowish-orange, green-yellow, and greenish-blue color code coordinates, respectively. Among them, self-activated InVO4 presents a good CRI of 71, which is already interesting for lighting applications. Furthermore, proper combination of the three phosphors may further improve the quality of the light emission; thus, they can be considered promising candidates for white LEDs or NIR emitting sources in the case of Yb-doped samples. The capability of converting UV photons in visible or NIR photons has also potential applications, improving the efficiency of silicon-based solar cells.

Figure 9

Figure 9. CIE diagram of InVO4, InNbO4:Tb, and InTaO4:Tb.

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Table 4. Chromaticity Coordinates (CIE), Correlated Color Temperature (CCT), Color Rendering Indices (CRI), and Color Emitted for InVO4, InNbO4:Tb, and InTaO4:Tb Samples
samplexyCCT (K)CRI (%)color
InVO40.430.47356171between warm white and neutral white
InNbO4:Tb0.260.33989537overcast sky, slightly blue-green
InTaO4:Tb0.340.58537927between daylight and sunlight

Conclusions

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Doping InVO4, InNbO4, and InTaO4 with Tb3+ or Yb3+ up to 0.2% at. concentration does not change the crystal structure and phonon frequencies of the host materials but provides peculiar optical excitation and emission properties in the visible and NIR spectral regions. The energy of the two first direct transitions was estimated in InVO4 at 3.7/4.1 eV in the Y → Y and Γ → Γ points in the BZ and in InNbO4 and InTaO4 at 4.7/5.3 and 5.6/6.1 eV in the Γ → Γ and Z → Z points in the BZ, respectively. InVO4, being a direct band gap semiconductor, showed a strong self-activated photoluminescence centered at 2.2 eV, in comparison with the indirect InNbO4 and InTaO4 semiconductors that showed weak self-activated signals at 2.2 and 2.9 eV. These signals were related to the irregular tetrahedral VO43– in InVO4 and the octahedra Nb(Ta)O43– in InNbO4 and InTaO4, respectively. The characteristic green (5D4 → 7FJ) and NIR (2F5/2 → 2F7/2) emission of Tb3+- and Yb3+-doped materials were analyzed, demonstrating to be potential candidates for applications as phosphors for white LED lighting and NIR emitting sources and improving the efficiency of silicon-based solar cells.

Experimental Section

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Undoped InMO4 (M = V5+, Nb5+, Ta5+) powders were prepared by a solid-state reaction following the method reported in previous works. (9−12) The composition and purity of the samples were confirmed by energy-dispersive X-ray spectroscopy analysis (EDAX) using a transmission electron microscope operated at 200 kV. The crystal structure was verified by powder (XRD) measurements and Raman spectroscopy (RS) measurements at room conditions, and the results were previously published. (9,11,12)
Polycrystalline InMO4:RE (M = V5+, Nb5+, Ta5+; RE = Tb3+, Yb3+) doped samples having Tb3+ or Yb3+ concentrations below 0.2% (see Table 1) were synthesized with the solid-state reaction method using predried powders of In2O3, Nb2O5, Ta2O5, V2O5, Yb2O3, and Tb4O7 (purity of >99.9%). The low doping concentration was selected following a previous work on Eu-doped InVO4 with the aim of obtaining good luminescence properties without affecting the crystal structure of the host material. For the doped tantalate and niobate, respective binary oxides were weighed in a stoichiometric ratio, thoroughly ground in a pestle and mortar, compacted by cold pressing into cylinders of 12.5 mm in diameter and 5 mm in height, and fired at 1100 °C for 24 h in a box-type resistive furnace followed by another heat treatment at 1200 °C for 24 h. For the vanadate, the first heating was carried out at 700 °C for 24 h followed by second heat treatment at 850 °C. All the samples are in a powder form.
Compositions were confirmed by EDS. The EDS measurements were performed using an Oxford Instruments X Max 80 EDS system attached to a Philips XL30ESEM. During EDS measurements, an accelerating voltage of 30 kV was employed. Since the samples are nonconducting, the ESEM was operated in environmental mode, where surface charge buildup on the sample was neutralized using water vapor. The accuracy of the EDS measurements was ensured by measurements of standard samples of known compositions in environmental mode. Each determination is the average of 32 runs, and at least three places were analyzed on each sample.
The crystal structures, as well as the possible structural modification introduced by the dopants, have been studied by XRD and RS techniques. For XRD measurements, a laboratory-based powder XRD using a rotating-anode generator (RAG) with a Mo (λ = 0.7107 Å) anode and a MAR345 area detector was used. RS measurements were excited with the 632.8 nm line of a He-Ne gas laser using a power of 2 mW. The scattered light was collected through a 50×/0.35 objective and sent to a Horiba Jobin Yvon LabRAM HR spectrometer with an edge filter cutting Raman signals below ∼50 cm–1. The signal was dispersed by a grating of 1200 grooves/mm and detected by a thermoelectrically cooled multichannel charge-coupled device detector enabling a spectral resolution below 2 cm–1.
For optical reflectance measurements in the UV–Vis–NIR at normal incidence, an optical setup consisting of a deuterium lamp, fused silica lenses, reflecting optics objectives, and a UV–Vis spectrometer was used. (34)
Photoluminescence excitation (PLE) and emission (PL) spectra were recorded by an Edinburgh Instruments FLS980 photoluminescence spectrometer. A continuous-wave xenon lamp was used as an excitation source for steady-state measurements, coupled to a double-grating monochromator for wavelength selection. The light emitted from the sample was collected by a double-grating monochromator and recorded by a photon counting R928P photomultiplier tube cooled at −20 °C in the visible spectral region and a R5509-73 photomultiplier tube cooled at −80 °C in the NIR spectral region.
The PLE signal was measured by following the emission signal at 550 nm (2.25 eV) and in the case of samples doped with Yb3+ also at 995 nm (1.25 eV). PL emission was measured in the visible region and in the case of samples doped with Yb3+ and also in the NIR region by using 310 nm (4 eV) excitation.
Time-resolved PL emission was obtained in multichannel scaling (MCS) mode, exciting the sample by a microsecond xenon flash lamp with a pulse duration of 1–2 μs and a repetition frequency of 10 Hz by using the same PMT detectors described above. The decay time was measured for the same signals as in PLE measurements. All abovementioned measurements were carried out at room temperature.

Computational Methods

The energy level scheme of Tb3+ and Yb3+ ions inside the crystals was modeled using a parametrized one-electron Hamiltonian in the 4fn ground configuration. The usual description includes both the several superimposed atomic interactions, which generate the 2S+1LJ multiplets, and the effect of the crystal field felt by the shielded 4f shell electron, when a rare earth ion incorporates into a solid host. The crystal field reflects the local symmetry of the RE location and is responsible for the breaking down of the 2S+1LJ degeneracy giving rise to the Stark levels. The total Hamiltonian can be expressed as (35)
(2)
Most of the parameters in the atomic Hamiltonian (EAVE, F(k), ζ4f, α, β, γ, M(j), P(k), and T(r)) were fixed to previously reported values in the present calculation. (35−37) Only EAVE, the Slater F2 parameter (for Tb3+), and the spin–orbit ζ4f parameter are slightly varied to properly estimate the position and separation of the multiplets involved in the observed transitions.
The number of nonvanishing parameters in the crystal field Hamiltonian depends on the point symmetry of the rare earth site in the host. For the studied structures, and due to the similarity of their ionic radius, we can assume that the RE3+ ion replaces the In3+ ions in sites with D2 local symmetry for InMO4 (M = Ta, Nb) compounds or C2h local symmetry for InVO4 crystals. In these symmetries, the degeneracies of the 2S+1LJ multiplets are completely lifted. By appropriated selection of the crystal field quantization axis, and a subsequent suitable rotation around the z axis to get B′22 = 0, both symmetries can be described by a crystal field Hamiltonian having only 14 nonvanishing parameters. The crystal field Hamiltonian takes the form
(3)
The CF parameters were calculated using a modified version of the simple overlap model (SOM) (38) that correlates the bond distance and the bond valence as in the usual bond valence model (details can be obtained from refs (14) and (39)). The crystallographic positions of the In3+ ion and its oxygen ligands obtained by ab initio calculations for every host matrix were used neglecting the small distortion of the In3+ site when occupied by a RE3+ ion. Following standard convention in the description of the crystal field interaction, the rotationally invariant crystal field strength parameter defined as (40)
(4)
has been also calculated to simplify the comparison of the crystal-field interaction in the different structures.
The obtained parameters and the energy level positions for each host and dopant ion are included in the Supporting Information. The energy level schemes obtained from these data has been used to analyze the optical spectra.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02862.

  • Crystal field parameters and crystal field strength values, calculated energy level positions for Tb3+ and Yb3+ ion in InNbO4, InTaO4, and InVO4, oxygen position and relative charge for oxygen ligands in In site used to obtain the crystal field parameters, graphical representations of the In local environment, and Miller indices, d-space, dispersion angle, and simulated X-ray diffraction intensity of InVO4, InNbO4, and InTaO4 (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Francesco Enrichi - Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-97187 Luleå, Sweden;  Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, via Torino 155, 30172 Venezia, Italy
    • Alberto Vomiero - Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-97187 Luleå, Sweden;  Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, via Torino 155, 30172 Venezia, Italy;  Orcidhttp://orcid.org/0000-0003-2935-1165
    • Juan E. Muñoz-Santiuste - Departamento de Física, MALTA Consolider Team, Escuela Politécnica Superior, Universidad Carlos III de Madrid, Avenida de la Universidad 30, E-28913 Leganés, Spain
    • Alka B. Garg - High Pressure and Synchrotron Radiation Physics Division and , Bhabha Atomic Research Centre, Mumbai 400085, India;  Orcidhttp://orcid.org/0000-0003-4050-8469
    • Ananthanarayanan Arvind - Process Development Division and , Bhabha Atomic Research Centre, Mumbai 400085, India
    • Francisco J. Manjón - Instituto de Diseño para la Fabricación y Producción Automatizada, MALTA Consolider Team, Universitat Politècnica de València, Camí de Vera s/n, 46022 València, Spain;  Orcidhttp://orcid.org/0000-0002-3926-1705
    • Alfredo Segura - Departamento de Física Aplicada-ICMUV, Universidad de Valencia, MALTA Consolider Team, Edificio de Investigación, C. Dr. Moliner 50, 46100 Burjassot, Spain;  Orcidhttp://orcid.org/0000-0002-9979-1302
    • Daniel Errandonea - Departamento de Física Aplicada-ICMUV, Universidad de Valencia, MALTA Consolider Team, Edificio de Investigación, C. Dr. Moliner 50, 46100 Burjassot, Spain;  Orcidhttp://orcid.org/0000-0003-0189-4221
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank the financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades, Spanish Research Agency (AEI), Generalitat Valenciana, and European Fund for Regional Development (ERDF, FEDER) under grants no. MAT2016-75586-C4-1/2-P, RTI2018-101020-B-I00, RED2018-102612-T (MALTA Consolier Team), and Prometeo/2018/123 (EFIMAT). P.B. acknowledges financial support from the Kempe Foundation and the Knut & Alice Wallenberg Foundation via a doctoral studentship. A.B.G. thanks the support provided by Universitat de Valencia to perform a research stay (Atracció de Talent, VLC-CAMPUS).

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    Zou, Z.; Ye, J.; Arakawa, H. Structural properties of InNbO4 and InTaO4: correlation with photocatalytic and photophysical properties. Chem. Phys. Lett. 2000, 332, 271,  DOI: 10.1016/S0009-2614(00)01265-3
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    Structural properties of InNbO4 and InTaO4: correlation with photocatalytic and photophysical properties
    Zou, Z.; Ye, J.; Arakawa, H.
    Chemical Physics Letters (2000), 332 (3,4), 271-277CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    The InTaO4 and InNbO4 photocatalysts were synthesized by solid-state reaction and characterized by powder x-ray diffraction and Rietveld structure refinement. These photocatalysts crystallize in the same wolframite structure and there are two kinds of octahedron, InO6 and NbO6(TaO6), in a unit cell. The octahedral TaO6 in InTaO4 slightly expands from that of NbO6 in InNbO4, leading to the longer lattice parameters in InTaO4. The band gaps of InTaO4 and InNbO4 were estd. to be 2.6 and 2.5 eV, resp. The difference is due to conduction band levels formed by Ta 5d in TaO6 and Nb 4d in NbO6, resulting in a little difference in photocatalytic activity of the two compds.
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    Balamurugan, C.; Vijayakumar, E.; Subramania, A. Synthesis and characterization of InNbO4 nanopowder for gas sensors. Talanta 2012, 88, 115– 120,  DOI: 10.1016/j.talanta.2011.10.017
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    Synthesis and characterization of InNbO4 nanopowder for gas sensors
    Balamurugan, C.; Vijayakumar, E.; Subramania, A.
    Talanta (2012), 88 (), 115-120CODEN: TLNTA2; ISSN:0039-9140. (Elsevier B.V.)
    Indium niobate (InNbO4) nanopowder was prepd. by a comparatively low temp. niobium citrate complex process. The prepd. InNbO4 was characterized by thermal anal., x-ray diffraction anal. (XRD), SEM, TEM, energy dispersive X-ray spectroscopy, diffuse reflectance spectroscopy (DRS), and impedance studies. The well cryst. monoclinic InNbO4 nanopowder was obtained at the calcination temp. of 600°. The av. particle diam. was 22 nm. The optical band gap is 2.66 eV. The temp. dependent cond. obeyed Arrhenius relation. The activation energy of the cond. process is 0.43 eV. The gas sensing behavior of the prepd. InNbO4 was studied by measuring the change in resistance of the sensor material as a function of various concns. of the test gases such as liq. petroleum gas (LPG), ammonia (NH3) and ethanol (C2H5OH) at their optimized operating temp. InNbO4 had a better sensitivity to LPG (0.97) and NH3 (0.70) gas than ethanol (0.46). The sensor responses of InNbO4 as a function of gas concns. and with recovery time were also studied.
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    Chen, L.; Liu, Y.; Lu, Z.; Zeng, D. Shape-controlled synthesis and characterization of InVO4 particles. J. Colloid Interface Sci. 2006, 295, 440– 444,  DOI: 10.1016/j.jcis.2005.09.051
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    Shape-controlled synthesis and characterization of InVO4 particles
    Chen, Limiao; Liu, Younian; Lu, Zhouguang; Zeng, Dongming
    Journal of Colloid and Interface Science (2006), 295 (2), 440-444CODEN: JCISA5; ISSN:0021-9797. (Elsevier)
    InVO4 powders in the shape of rod-like, cubic-like, irregular particle and brick-like have been successfully synthesized by a hydrothermal method in the presence of different org. additives, namely, CTAB, SDS, PVP, and EDTA. The products were characterized by X-ray diffraction, transmission electron microscopy (TEM) and scanning electron microscope (SEM). From the exptl. results, it has been revealed that there is optimal pH range ( pH ≤ 7.0 ) for the formation of pure InVO4 and the org. additives play a key role in the formation of the products with different morphologies and sizes. Gas sensing behavior of this material to ethanol and ammonia has been studied. The gas sensors constructed by the InVO4 particles with different morphologies show different sensitivity to ethanol. It is also found that the sensor based on InVO4 is more sensitive to ethanol than to ammonia.
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  9. 9
    Errandonea, D.; Popescu, C.; Garg, A. B.; Botella, P.; Martinez-García, D.; Pellicer-Porres, J.; Rodríguez-Hernández, P.; Muñoz, A.; Cuenca-Gotor, V.; Sans, J. A. Pressure-induced phase transition and band-gap collapse in the wide-band-gap semiconductor InTaO4. Phys. Rev. B 2016, 93, 035204  DOI: 10.1103/PhysRevB.93.035204
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    Pressure-induced phase transition and band-gap collapse in the wide-band-gap semiconductor InTaO4
    Errandonea, D.; Popescu, C.; Garg, A. B.; Botella, P.; Martinez-Garcia, D.; Pellicer-Porres, J.; Rodriguez-Hernandez, P.; Munoz, A.; Cuenca-Gotor, V.; Sans, J. A.
    Physical Review B (2016), 93 (3), 035204/1-035204/12CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)
    A pressure-induced phase transition, assocd. with an increase of the coordination no. of In and Ta, is detected beyond 13 GPa in InTaO4 by combining synchrotron x-ray diffraction and Raman measurements in a diamond-anvil cell with ab initio calcns. High-pressure optical-absorption measurements were also carried out. The high-pressure phase has a monoclinic structure that shares the same space group with the low-pressure phase (P2/c). The structure of the high-pressure phase can be considered as a slight distortion of an orthorhombic structure described by space group Pcna. The phase transition occurs together with a unit-cell vol. collapse and an electronic band-gap collapse obsd. by expts. and calcns. Addnl., a band crossing is found to occur in the low-pressure phase near 7 GPa. The pressure dependence of all the Raman-active modes is reported for both phases as well as the pressure dependence of unit-cell parameters and the equations of state. Calcns. also provide information on IR-active phonons and bond distances. These findings provide insights into the effects of pressure on the phys. properties of InTaO4.
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    Botella, P.; Errandonea, D.; Garg, A. B.; Rodríguez-Hernandez, P.; Muñoz, A.; Achary, S. N.; Vomiero, A. High-pressure characterization of the optical and electronic properties of InVO4, InNbO4, and InTaO4. SN Appl. Sci. 2019, 1, 389,  DOI: 10.1007/s42452-019-0406-7
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    Garg, A. B.; Errandonea, D.; Popescu, C.; Martinez-García, D.; Pellicer-Porres, J.; Rodríguez-Hernández, P.; Muñoz, A.; Botella, P.; Cuenca-Gotor, V. P.; Sans, J. A. Pressure-Driven Isostructural Phase Transition in InNbO4: In Situ Experimental and Theoretical Investigations. Inorg. Chem. 2017, 56, 5420,  DOI: 10.1021/acs.inorgchem.7b00437
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    11
    Pressure-Driven Isostructural Phase Transition in InNbO4: In Situ Experimental and Theoretical Investigations
    Garg, Alka B.; Errandonea, Daniel; Popescu, Catalin; Martinez-Garcia, Domingo; Pellicer-Porres, Julio; Rodriguez-Hernandez, Placida; Munoz, Alfonso; Botella, Pablo; Cuenca-Gotor, Vanesa P.; Sans, Juan Angel
    Inorganic Chemistry (2017), 56 (9), 5420-5430CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The high-pressure behavior of technol. important visible-light photocatalytic semiconductor InNbO4, adopting a monoclinic wolframite-type structure at ambient conditions, was investigated using synchrotron-based X-ray diffraction, Raman spectroscopic measurements, and first-principles calcns. The exptl. results indicate the occurrence of a pressure-induced isostructural phase transition in the studied compd. beyond 10.8 GPa. The large vol. collapse assocd. with the phase transition and the coexistence of two phases obsd. over a wide range of pressure shows the nature of transition to be first-order. There is an increase in the oxygen anion coordination no. around In and Nb cations from six to eight at the phase transition. The ambient-pressure phase has been recovered on pressure release. The exptl. pressure-vol. data when fitted to a Birch-Murnaghan equation of states yields the value of ambient pressure bulk modulus as 179(2) and 231(4) GPa for the low- and high-pressure phases, resp. The pressure dependence of the Raman mode frequencies and Gruneisen parameters was detd. for both phases by exptl. and theor. methods. The same information is obtained for the IR modes from first-principles calcns. Results from theor. calcns. corroborate the exptl. findings. They also provide information on the compressibility of interat. bonds, which is correlated with the macroscopic properties of InNbO4.
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  12. 12
    Errandonea, D.; Gomis, O.; García-Domene, B.; Pellicer-Porres, J.; Katari, V.; Achary, S. N.; Tyagi, A. K.; Popescu, C. New Polymorph of InVO4: A High-Pressure Structure with Six-Coordinated Vanadium. Inorg. Chem. 2013, 52, 12790,  DOI: 10.1021/ic402043x
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    12
    New Polymorph of InVO4: A High-Pressure Structure with Six-Coordinated Vanadium
    Errandonea, Daniel; Gomis, Oscar; Garcia-Domene, Braulio; Pellicer-Porres, Julio; Katari, Vasundhara; Achary, S. Nagabhusan; Tyagi, Avesh K.; Popescu, Catalin
    Inorganic Chemistry (2013), 52 (21), 12790-12798CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A new wolframite-type polymorph of InVO4 is identified under compression near 7 GPa by in situ high-pressure (HP) X-ray diffraction (XRD) and Raman spectroscopic investigations on the stable orthorhombic InVO4. The structural transition is accompanied by a large vol. collapse (ΔV/V = -14%) and a drastic increase in bulk modulus (from 69 to 168 GPa). Both techniques also show the existence of a third phase coexisting with the low- and high-pressure phases in a limited pressure range close to the transition pressure. XRD studies revealed a highly anisotropic compression in orthorhombic InVO4. In addn., the compressibility becomes nonlinear in the HP polymorph. The vol. collapse in the lattice is related to an increase of the polyhedral coordination around the vanadium atoms. The transformation is not fully reversible. The drastic change in the polyhedral arrangement obsd. at the transition is indicative of a reconstructive phase transformation. The HP phase here found is the only modification of InVO4 reported to date with 6-fold coordinated vanadium atoms. Finally, Raman frequencies and pressure coeffs. in the low- and high-pressure phases of InVO4 are reported.
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  13. 13
    Errandonea, D.; Tu, C.; Jia, G.; Martín, I. R.; Rodríguez-Mendoza, U. R.; Lahoz, F.; Torres, M. E.; Lavín, V. Effect of pressure on the luminescence properties of Nd3+ doped SrWO4 laser crystal. J. Alloys Compd. 2008, 451, 212– 214,  DOI: 10.1016/j.jallcom.2007.04.180
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    Effect of pressure on the luminescence properties of Nd3+ doped SrWO4 laser crystal
    Errandonea, D.; Tu, Chaoyang; Jia, Guohua; Martin, I. R.; Rodriguez-Mendoza, U. R.; Lahoz, F.; Torres, M. E.; Lavin, V.
    Journal of Alloys and Compounds (2008), 451 (1-2), 212-214CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    The luminescence spectra of the 4F3/2→4I9/2 transition of Nd3+ ions in a SrWO4 crystal were analyzed as a function of pressure at room temp. Expts. were performed in a diamond-anvil cell up to 13 GPa. At ∼10 GPa some changes in the emission spectra were obsd. which are attributed to a structural phase transition of the SrWO4 matrix. These results are in good agreement with a previous paper, in which in a pure SrWO4 matrix a scheelite to fergusonite phase transition is found around 10.5 GPa. Also, with increasing pressure, the decay curves from the 4F3/2 are nonexponential and faster indicating that the energy transfer processes between Nd3+ ions are important.
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    Muñoz-Santiuste, J. E.; Lavín, V.; Rodríguez-Mendoza, U. R.; Ferrer-Roca, C.; Errandonea, D.; Martínez-García, D.; Rodríguez-Hernández, P.; Muñoz, A.; Bettinelli, M. Experimental and theoretical study on the optical properties of LaVO4 crystals under pressure. Phys. Chem. Chem. Phys. 2018, 20, 27314,  DOI: 10.1039/C8CP04701D
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    14
    Experimental and theoretical study on the optical properties of LaVO4 crystals under pressure
    Munoz-Santiuste, J. E.; Lavin, V.; Rodriguez-Mendoza, U. R.; Ferrer-Roca, Ch.; Errandonea, D.; Martinez-Garcia, D.; Rodriguez-Hernandez, P.; Munoz, A.; Bettinelli, M.
    Physical Chemistry Chemical Physics (2018), 20 (43), 27314-27328CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We report optical absorption and luminescence measurements in pure and trivalent neodymium (Nd3+) doped LaVO4 crystals up to 25 GPa. Nd3+ luminescence has been employed as a tool to follow the structural changes in the crystal. We also present band-structure and crystal-field calcns. that provide the theor. framework to accurately explain the obsd. exptl. results. In particular, both optical absorption and luminescence measurements evidence that a phase transition takes place close to 12 GPa. They also provide information on the pressure dependence of the band-gap as well as the emission lines under compression. We found drastic changes in the optical properties of LaVO4 when the phase transition to a BaWO4-II structure occurs, which can be related to changes in the coordination no. of vanadium ions and in the local sites of Nd3+. Reported results are analyzed in comparison with those of previous X-ray diffraction and Raman expts., as well as with the features of related compds. For the first time, a consistent picture is reported explaining the behavior of the optical and electronic properties of LaVO4 at high-pressures.
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    Yoo, S. H.; Kum, J. M.; Cho, S. O. Tuning the electronic band structure of PCBM by electron irradiation. Nanoscale Res. Lett. 2011, 6, 545,  DOI: 10.1186/1556-276X-6-545
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    Tuning the electronic band structure of PCBM by electron irradiation
    Yoo, Seung Hwa; Kum, Jong Min; Cho, Sung Oh
    Nanoscale Research Letters (2011), 6 (1), 545/1-545/7CODEN: NRLAAD; ISSN:1556-276X. (Springer)
    Tuning the electronic band structures such as band-edge position and bandgap of org. semiconductors is crucial to maximize the performance of org. photovoltaic devices. We present a simple yet effective electron irradn. approach to tune the band structure of [6, 6]-phenyl-C61-butyric acid Me ester (PCBM) that is the most widely used org. acceptor material. We have found that the LUMO (LUMO) level of PCBM up-shifts toward the vacuum energy level, while the HOMO (HOMO) level down-shifts when PCBM is electron-irradiated. The shift of the HOMO and the LUMO levels increases as the irradiated electron fluence increases. Accordingly, the band-edge position and the bandgap of PCBM can be controlled by adjusting the electron fluence. Characterization of electron-irradiated PCBM reveals that the variation of the band structure is attributed to the mol. structural change of PCBM by electron irradn.
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    Tapasztó, L.; Dobrik, G.; Nemes-Incze, P.; Vertesy, G.; Lambin, P.; Biró, L. P. Tuning the electronic structure of graphene by ion irradiation. Phys. Rev. B 2008, 78, 233407,  DOI: 10.1103/PhysRevB.78.233407
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    16
    Tuning the electronic structure of graphene by ion irradiation
    Tapaszto, L.; Dobrik, G.; Nemes-Incze, P.; Vertesy, G.; Lambin, Ph.; Biro, L. P.
    Physical Review B: Condensed Matter and Materials Physics (2008), 78 (23), 233407/1-233407/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Mech. exfoliated graphene layers deposited on SiO2 substrate were irradiated with Ar+ ions in order to exptl. study the effect of at. scale defects and disorder on the low-energy electronic structure of graphene. The irradiated samples were investigated by scanning tunneling microscopy and spectroscopy measurements, which reveal that defect sites, besides acting as scattering centers for electrons through local modification of the on-site potential, also induce disorder in the hopping amplitudes. The most important consequence of the induced disorder is the substantial redn. in the Fermi velocity, revealed by bias-dependent imaging of electron-d. oscillations obsd. near defect sites.
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    Shih, H.-R.; Liu, K.-T.; Teoh, L.-G.; Wei, L.-K.; Chang, Y.-S. Synthesis and photoluminescence properties of (La,Pr) co-doped InVO4 phosphor. Microelectron. Eng. 2015, 148, 10– 13,  DOI: 10.1016/j.mee.2015.07.007
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    17
    Synthesis and photoluminescence properties of (La, Pr) co-doped InVO4 phosphor
    Shih, Hung-Rung; Liu, Kuan-Ting; Teoh, Lay-Gaik; Wei, Li-Kai; Chang, Yee-Shin
    Microelectronic Engineering (2015), 148 (), 10-13CODEN: MIENEF; ISSN:0167-9317. (Elsevier B.V.)
    (La3 +, Pr3 +) co-doped InVO4 phosphors were prepd. using a sol-gel method. For the (In1 - xPrx)VO4 (x = 0-0.1) system, X-ray powder diffraction (XRD) patterns show that all of the peaks are attributed to the orthorhombic phase, and the SEM (SEM) images show that there is no obvious difference for the particle morphol. between various Pr3 + ions doping. No emission peak characteristics of Pr3 + ions were obsd. under an excitation of 326 nm. In the (In0.97 - yLayPr0.03)VO4 (y = 0-0.97) system, XRD patterns indicated that the crystal structure changed from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure when the In3 + ions were gradually substituted by the La3 + ions. The location of excitation peaks shifted to short wavelength area (blue shift), and the characteristic f-f transition of Pr3 + ion was obsd. With increasing La3 + ion concn., a series of emission peaks assigned to the 3P0 → 3H4,5,6, 1D2 → 3H4, and 3P0 → 3F2 characteristic transitions of Pr3 + ion were obsd. in the emission spectra under an excitation of 315 nm, and this indicated that the La3 + ion can act as a sensitizer for the (In0.97 - yLayPr0.03)VO4 phosphor. The Commission Internationale de I'Eclairage (CIE) color chromaticity coordinates of (In0.97 - yLayPr0.03)VO4 phosphors vary with the La3 + ion concn. from the yellowish light region, though the green-yellow and finally to the near-white light area as the La3 + ion concn. increased.
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    Lu, M.; Li, Q.; Zhou, C.; Zhang, C.; Shi, H. Effects of nonmetal elements doping on the electronic structures of InNbO4: first-principles calculations. Mater. Res. Express 2018, 5, 075505  DOI: 10.1088/2053-1591/aace04
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    18
    Effects of nonmetal elements doping on the electronic structures of InNbO4: first-principles calculations
    Lu, Mengfei; Li, Qiaoqiao; Zhou, Changping; Zhang, Chengliang; Shi, Haifeng
    Materials Research Express (2018), 5 (7), 075505/1-075505/8CODEN: MREAC3; ISSN:2053-1591. (IOP Publishing Ltd.)
    To explore the effects of nonmetal elements mono-doping, the electronic structures of (B, C, P, F) doped InNbO4 have been investigated systematically based on the first-principles calcns. The obtained results revealed that the band edges of (B, C, P) doped InNbO4 slightly expanded while the band gap of F doped InNbO4 barely changed. Meanwhile, different p orbital states were introduced into the band gaps of InNbO4 after (B, C, P) doping, which was in favor of extending the absorption spectra to the visible-light region. Among these nonmetal doping systems, it was noted that the band edge potentials of InNbO4 satisfied the requirement for water splitting in C doped InNbO4. The localized C 2p states could perform as intermediate levels to reduce the transition energy of photo-induced electrons. Therefore, we put forward that C atom should be a suitable dopant for single anion doping InNbO4, which will provide a guideline for developing doping InNbO4 with visible-light sensitive reactions.
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    Song, Y.; Sun, Z.; Wu, Y.; Chai, Z.; Wang, X. Investigation of the Preferential Doping Site and Regulating on the Visible Light Response and Redox Performance for Fe- and/or La Doped InNbO4. Inorg. Chem. 2018, 57, 8558– 8567,  DOI: 10.1021/acs.inorgchem.8b01287
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    19
    Investigation of the Preferential Doping Site and Regulating on the Visible Light Response and Redox Performance for Fe- and/or La-Doped InNbO4
    Song, Yanyong; Sun, Zhebin; Wu, Yuhang; Chai, Zhanli; Wang, Xiaojing
    Inorganic Chemistry (2018), 57 (14), 8558-8567CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The preferential doping site, visible light response, and redox potential of Fe- and/or La-doped InNbO4 (INO) were studied using 1st-principles d. functional theory. Eight designed doping models, including Fe and/or La doping at In or/and Nb sites of INO are constructed, resp. It was found that Fe-doping and Fe,La-codoping to substitute In into an INO cell are energetically favorable, confirming that the steric hindrance plays a vital role for the selective doping site than the charge of the dopants. Fe doping always formed 2 impurity bands between the conduction and valence bands, originated from Fe 3d state, inferring the well visible light response. The presence of La has a specific regulation effects for Fe doping although the energy levels of the single La-doped models were completely similar to those of the undoped INO. The electron exchange between La and Fe dopants results in the significant interaction for codoping INO. Importantly, by doping La into INO cell, the redox potentials of Fe-doped INO could be well-regulated. The band potential moved to the more pos. energy level of the models Fe-doped at Nb sites, while it shifted to the more neg. level if Fe was doped at In site of La-INO. This study may provide the guidance for the designative dopants to construct the photocatalyst with stable, visible response, and good redox performance.
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    Rakesh, K.; Khaire, S.; Bhange, D.; Dhanasekaran, P.; Deshpande, S. S.; Awate, S. V.; Gupta, N. M. Role of doping-induced photochemical and microstructural properties in the photocatalytic activity of InVO4 for splitting of water. J. Mater. Sci. 2011, 46, 5466– 5476,  DOI: 10.1007/s10853-011-5489-5
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    20
    Role of doping-induced photochemical and microstructural properties in the photocatalytic activity of InVO4 for splitting of water
    Rakesh, K.; Khaire, S.; Bhange, D.; Dhanasekaran, P.; Deshpande, S. S.; Awate, S. V.; Gupta, N. M.
    Journal of Materials Science (2011), 46 (16), 5466-5476CODEN: JMTSAS; ISSN:0022-2461. (Springer)
    We report in this paper on microstructural, optical and photocatalytic properties of single-phase indium orthovanadates, as a function of doping at lattice sites. The UV-visible spectra of these samples exhibited intense UV-region bands at 250 and 350 nm, besides broad absorption band in visible region (350-700 nm). The wavelength at absorption edge and the intensity of visible absorption showed considerable increase on doping of an impurity, particularly at V or O lattice sites. Also, the samples gave rise to blue-green photoluminescence emission, with overriding bands at ca. 420, 450, 460 and 485 nm, on excitation at 240-420 nm wavelengths. The intensity of these fluorescence bands varied with excitation wavelength and impurity content of a sample. In deviation with several earlier studies, only oxygen and no hydrogen were produced during photocatalytic splitting of water, in the expts. conducted under visible light (>395 nm) and at a pH of ∼6.5. The O2 yield depended on the dispersed metal co-catalyst, impurity content and the addn. of methanol as sacrificial reagent. On the other hand, small quantities of hydrogen and no oxygen were evolved on UV-irradn. of pure water using metal/InVO4. These results are ascribed to flat band potentials and the doping-induced inter-band donor and acceptor charge trapping states of InVO4, the presence of which is revealed by XRD, luminescence and XPS studies. Our study also confirms that the onset of absorption edge may not necessarily correspond to band-to-band energy gap of a semiconducting material. This accounts for some anomalous band gap energies reported earlier for InVO4.
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    Wetchakun, N.; Wanwaen, P.; Phanichphant, S.; Wetchakun, K. Influence of Cu doping on the visible-light-induced photocatalytic activity of InVO4. RSC Adv. 2017, 7, 13911,  DOI: 10.1039/C6RA27138C
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    Influence of Cu doping on the visible-light-induced photocatalytic activity of InVO4
    Wetchakun, Natda; Wanwaen, Pimonrat; Phanichphant, Sukon; Wetchakun, Khatcharin
    RSC Advances (2017), 7 (23), 13911-13918CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The photocatalytic degrdn. of methylene blue (MB) in the presence of pure InVO4 or a 0.5-5.0 mol% Cu-doped InVO4 composite under visible light irradn. (λ ≥ 400 nm) was studied in this research. The structural and photophys. properties of the as-prepd. samples in the photocatalytic degrdn. process were investigated. The doping of InVO4 with a Cu photocatalyst results in wide absorption in the visible-light region and superior visible-light-driven photocatalytic activities in the degrdn. of MB. The results indicate that the InVO4 sample doped with 1.0 mol% of Cu shows the highest photocatalytic activity. The enhanced photocatalytic activity was attributed to the copper ions acting as trapping sites, facilitating the sepn. of charge carriers. The main active species for the degrdn. of MB were investigated to explain the enhancement of the photocatalytic performance of Cu-doped InVO4. A possible photocatalytic degrdn. pathway for aq. MB dye and a charge transfer mechanism for Cu-doped InVO4 were proposed.
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    Malingowski, A. C.; Stephens, P. W.; Huq, A.; Huang, Q.; Khalid, S.; Khalifah, P. G. Substitutional Mechanism of Ni into the Wide-Band-Gap Semiconductor InTaO4 and Its Implications for Water Splitting Activity in the Wolframite Structure Type. Inorg. Chem. 2012, 51, 6096– 6103,  DOI: 10.1021/ic202715c
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    Substitutional Mechanism of Ni into the Wide-Band-Gap Semiconductor InTaO4 and Its Implications for Water Splitting Activity in the Wolframite Structure Type
    Malingowski, Andrew C.; Stephens, Peter W.; Huq, Ashfia; Huang, Qingzhen; Khalid, Syed; Khalifah, Peter G.
    Inorganic Chemistry (2012), 51 (11), 6096-6103CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The mechanism of Ni substitution into the oxide semiconductor InTaO4 has been studied through a combination of structural and spectroscopic techniques, providing insights into its previously reported photoactivity. Magnetic susceptibility and x-ray absorption near-edge spectroscopy (XANES) measurements demonstrate that nickel is divalent within the host lattice. The combined refinement of synchrotron x-ray and neutron powder diffraction data indicates that the product of Ni doping has the stoichiometry of (In1-xNi2x/3Tax/3)TaO4 with a soly. limit of x ≈ 0.18, corresponding to 12% Ni on the In site. Single-phase samples were only obtained at synthesis temps. of 1150° C or higher due to the sluggish reaction mechanism that is hypothesized to result from small free energy differences between (In1-xNi2x/3Tax/3)TaO4 compds. with different x values. Undoped InTaO4 is shown to have an indirect band gap of 3.96 eV, with direct optical transitions becoming allowed at photon energies in excess of 5.1 eV. Very small band-gap redns. (less than 0.2 eV) result from Ni doping, and the origin of the yellow color of (In1-xNi2x/3Tax/3)TaO4 compds. instead results from a weak 3A2g → 3T1g internal d → d transition not assocd. with the conduction or valence band that is common to oxide compds. with Ni2+ in an octahedral environment.
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    Su, L.; Fan, X.; Cai, G.; Jin, Z. Tunable luminescence properties and energy transfer of Tm3+, Dy3+, and Eu3+ co-activated InNbO4 phosphors for warm-white-lighting. Ceram. Int. 2016, 42, 15994– 16006,  DOI: 10.1016/j.ceramint.2016.07.105
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    Tunable luminescence properties and energy transfer of Tm3+, Dy3+, and Eu3+ co-activated InNbO4 phosphors for warm-white-lighting
    Su, Liumei; Fan, Xing; Cai, Gemei; Jin, Zhanpeng
    Ceramics International (2016), 42 (14), 15994-16006CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
    A series of Dy3+/Tm3+ singly-doped, Dy3+/Tm3+ co-doped, and Dy3+/Tm3+/Eu3+ tri-doped InNbO4 phosphors have been synthesized by a high-temp. solid-state reaction method. Powder X-ray diffraction (XRD), structure refinement, UV-visible diffuse reflectance (UV-DR) spectrum, photoluminescence excitation (PLE) and emission (PL) spectrum, decay lifetimes and CIE chromaticity coordinate were employed to understand the origins of luminescence properties for synthesized phosphors. Rare-earth activators were detd. to be completely dissolved into the host lattice and occupied the 2e sites deviated from an inversion center by virtue of XRD, structure refinement and photoluminescence features. The photoluminescence excitation spectra of phosphors exhibited a broad excitation band ascribed to charge-transfer of O2--Nb5+, ranging from 200 to 300 nm, and some characteristic excitation peaks assocd. with energy level transition of RE ions. Under the excitation of UV light, phosphors presented characteristic emission originating from f-f transition within the 4f configuration of RE ions. With increasing Dy3+ concn., emission colors of InNbO4:Tm3+, Dy3+ phosphors can be appropriately tuned from blue to yellow, including almost all the white light region, by means of energy transfer from Tm3+ to Dy3+. Energy transfer rates and efficiencies were calcd. according to decay lifetimes and energy transfer mechanism followed a resonant type dipole-dipole interaction with crit. distance of 22.23 Å between Tm3+ and Dy3+. By compensating a red component of Eu3+ ion, single-component warm-white-light emitting was realized in InNbO4:Tm3+,Dy3+,Eu3+ phosphors. They might be promising candidates for color-tunable light-conversion components in the fabrication of UV-pumped warm-white-light emitting diodes (WLEDs) for solid-state lighting. Relationship of crystal structures and luminescence properties for InNbO4 and YNbO4 phosphors is as well explicated based on structural anal.
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    Tang, A.; Zhang, D. F.; Yang, L. Synthesis and luminescence properties of novel red emitting phosphor InNbO4:Eu3+ for white light emitting diodes. Russ. Chem. Bull. 2012, 61, 2172,  DOI: 10.1007/s11172-012-0304-2
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    Synthesis and luminescence properties of novel red-emitting phosphor InNbO4:Eu3+ for white-light emitting diodes
    Tang, A.; Zhang, D. F.; Yang, Liu
    Russian Chemical Bulletin (2012), 61 (11), 2172-2174CODEN: RCBUEY; ISSN:1066-5285. (Springer)
    Red-emitting phosphor InNbO4:Eu3+ was synthesized by the solid-state reaction. Its crystal structure, particle size distribution, and luminescence properties were studied. The powder X-ray diffraction pattern shows that pure InNbO4:Eu3+ was obtained. According to the spectra obtained, this phosphor can efficiently be excited with the light at wavelengths of 394 and 466 nm to emit the strong red radiation at 612 nm due to the 5D0→7F2 transition of Eu3+. The best results were obtained at the concn. of the Eu3+ dopant equal to 4 mol.%. The chromaticity parameters of InNbO4:0.04Eu3+ are close to stds. of the National Television Std. Committee. Thus, InNbO4:Eu3+ is a promising red-emitting phosphor for white-light emitting diodes.
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    Errandonea, D.; Ruiz-Fuertes, J. A brief review of the effects of pressure on wolframite-type oxides. Crystals 2018, 8, 71,  DOI: 10.3390/cryst8020071
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    A brief review of the effects of pressure on wolframite-type oxides
    Errandonea, Daniel; Ruiz-Fuertes, Javier
    Crystals (2018), 8 (2), 71/1-71/19CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
    In this article, we review the advances that have been made on the understanding of the high-pressure (HP) structural, vibrational, and electronic properties of wolframite-type oxides since the first works in the early 1990s. Mainly tungstates, which are the best known wolframites, but also tantalates and niobates, with an isomorphic ambient-pressure wolframite structure, have been included in this review. Apart from estg. the bulk moduli of all known wolframites, the cation-oxygen bond distances and their change with pressure have been correlated with their compressibility. The compn. variations of all wolframites have been employed to understand their different structural phase transitions to post-wolframite structures as a response to high pressure. The no. of Raman modes and the changes in the band-gap energy have also been analyzed in the basis of these compositional differences. The reviewed results are relevant for both fundamental science and for the development of wolframites as scintillating detectors. The possible next research avenues of wolframites under compression have also been evaluated.
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    Baran, E. J. Materials belonging to the CrVO4 structure type: preparation, crystal chemistry and physicochemical properties. J. Mater. Sci. 1998, 33, 2479,  DOI: 10.1023/A:1004380530309
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    Materials belonging to the CrVO4 structure type: preparation, crystal chemistry and physicochemical properties
    Baran, E. J.
    Journal of Materials Science (1998), 33 (10), 2479-2497CODEN: JMTSAS; ISSN:0022-2461. (Chapman & Hall)
    A review, with ∼220 refs. Crystallog. data of vanadates, phosphates, chromates, sulfates and selenates belonging to the CrVO4 (β-CrPO4) structure type and of some of its most important polymorphs, are reported. Characteristic structural peculiarities and the effect of pressure on the phase transformations are discussed. A definitive stability field for this structure type could be established. Synthetic procedures for the prepn. of the different compds. adopting this and closely related structures are presented and its thermal, spectroscopic and magnetic properties are discussed. A brief overview on the most important characteristics of materials adopting the α-CrPO4 structure is also presented.
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    López-Moreno, S.; Rodríguez-Hernández, P.; Muñoz, A.; Errandonea, D. First-Principles Study of InVO4 under Pressure: Phase Transitions from CrVO4 to AgMnO4-Type Structure. Inorg. Chem. 2017, 56, 2697– 2711,  DOI: 10.1021/acs.inorgchem.6b02867
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    First-Principles Study of InVO4 under Pressure: Phase Transitions from CrVO4- to AgMnO4-Type Structure
    Lopez-Moreno, Sinhue; Rodriguez-Hernandez, Placida; Munoz, Alfonso; Errandonea, Daniel
    Inorganic Chemistry (2017), 56 (5), 2697-2711CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    First-principles calcns. were carried out to study the InVO4 compd. under pressure. Total energy calcns. were performed to analyze the structural behavior of the exptl. known polymorphs of InVO4: α-MnMoO4-type (I), CrVO4-type (III), and wolframite (V). The results about the stability of this compd. beyond the pressures reached by expts. are presented. Some new high-pressure phases are proposed from the study of 13 possible candidates. The quasi-harmonic approxn. was used to calc. the sequence of phase transitions at 300 K: CrVO4-type, III (the transition pressure is given in parentheses) → wolframite, V (4.4 GPa) → raspite, VI (28.1 GPa) → AgMnO4-type, VII (44 GPa). Equations of state and phonon frequencies as a function of pressure were calcd. for the studied phases. To det. the stability of each phase, the authors also report the phonon dispersion along the Brillouin zone and the phonon d. of states for the most stable polymorphs. The electronic band structure for the low- and high-pressure phases for the studied polymorphs is presented as well as the pressure evolution of the band gap by using the HSE06 hybrid functional.
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    Blasse, G.; Bril, A. Luminescence of Phosphors Based on Host Lattices ABO4 (A is Sc, In; B is P, V, Nb). J. Chem. Phys. 1969, 50, 2974,  DOI: 10.1063/1.1671493
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    Luminescence of phosphors based on host lattices ABO4 (A is Sc, In; B is P, V, Nb)
    Blasse, G.; Bril, Alfred
    Journal of Chemical Physics (1969), 50 (7), 2974-80CODEN: JCPSA6; ISSN:0021-9606.
    The luminescence of the unactivated and Bi3+- and Eu3+-activated compds. ABO4 (A is Sc, In; B is P, V, Nb) is reported and compared with the luminescence of the corresponding rare-earth and Y compds. The ScNbO4 shows a weak, blue emission, ScVO4 a blue-green, and InVO4 a green emission. The other host lattices show no or only a very weak emission at room temp. The color of the vanadate emission depends strongly on the choice of A. This was studied by replacing part of the A ions by other ions or a combination of ions. The shift of the vanadate emission is ascribed to 2 effects, viz., metal-metal interaction (e.g., Bi3+-VO4) and the effect of the size of the metal ions at the A sites (e.g., Sc3+). The Eu3+-activated phosphors show some interesting features. The intensity of the forced elec. dipole emission from the Eu3+ ion seems to depend strongly on the position of the lowest strong absorption band. In the zircon structure the 5D0-7F2 emission is hypersensitive to the surroundings, although no linear crystal-field component can be present. Both (In, Eu)PO4 and (In, Eu)VO4 show an orange (5D0-7F1) emission due to the presence of inversion symmetry at the In3+ site.
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    Tang, A.; Gu, L.; Shao, F.; Liu, X.; Zhao, Y.; Chen, H.; Zhang, H. Influence of Bi3+ content on photoluminescence of InNbO4:Eu3+, Bi3+ for white light-emitting diodes. Mater. Sci.-Pol. 2017, 35, 435– 439,  DOI: 10.1515/msp-2017-0053
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    Influence of Bi3+ content on photoluminescence of InNbO4:Eu3+,Bi3+ for white light-emitting diodes
    Tang, An; Gu, Liduo; Shao, Fengxiang; Liu, Xidong; Zhao, Yongtao; Chen, Haijun; Zhang, Hongsong
    Materials Science-Poland (2017), 35 (2), 435-439CODEN: MSACDV; ISSN:2083-134X. (Walter de Gruyter GmbH)
    A series of red-emitting phosphors InNbO4:Eu3+,Bi3+ was prepd. by a high temp. solid-state reaction. The structure, size distribution and luminescence properties of the phosphors were resp. characterized by X-ray diffraction (XRD), laser particle size and mol. fluorescence spectrometer. The XRD results indicate that the phase-pure samples have been obtained and the crystal structure of the host has not changed under the Eu3+ and Bi3+ co-doping. The test of size distribution shows that the phosphor has a normal size distribution. The excitation spectra illustrate that the dominant sharp peaks are located at 394 nm (7F0→5L6) and 466 nm (7F0→5D2). Meanwhile, the emission spectra reveal that the phosphors excited by the wavelength of 394 nm or 466 nm have an intense red-emission line at 612 nm owing to the 5D0→7F2 transition of Eu3+. Bi3+ doping has not changed the peak positions except the photoluminescence intensity. The emission intensity is related to Bi3+ concn., and it is up to the max. when the Bi3+-doping concn. is 4 mol%. Due to good photoluminescence properties of the phosphor, the InNbO4:0.04Eu3+,0.04Bi3+ may be used as a red component for white light-emitting diodes.
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    Shia, Z.-R.; Chen, H.-L.; Tsai, Y.-Y.; Wu, S.; Chang, Y.-S. Synthesis and Photoluminescence Properties of InVO4:Eu3+ Phosphors Prepared using Sol–Gel Method. ECS Trans. 2010, 28, 145– 154,  DOI: 10.1149/1.3367220
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    Brixner, L. H.; Chen, H.-Y. On the structural and luminescent properties of the InTa1–xNbxO4 system. Mater. Res. Bull. 1980, 15, 607– 612,  DOI: 10.1016/0025-5408(80)90140-3
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    On the structural and luminescent properties of the indium tantalum niobate (InTa1-xNbxO4) system
    Brixner, L. H.; Chen, H. Y.
    Materials Research Bulletin (1980), 15 (5), 607-12CODEN: MRBUAC; ISSN:0025-5408.
    InNbO4 and InTaO4 crystallize in the monoclinic space group P2/a of the wolframite structure and unlimited solid solns. are formed over the entire range of x. The cell vol. decreases with increasing Nb content. Both the niobate and tantalate are UV-excitable blue-emitting phosphors with the emission centered around 403 nm in a broad band of ∼85 nm half width. In the pure state, InTaO4 is a more efficient phosphor than InNbO4, max. quantum yield being obsd. at x = 0.075. Eu3+- and Tb3+-doped InTaO4 also exhibit luminescent properties.
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    Shen, J.; Yang, H.; Shen, Q.; You, Z. Synthesis and Characterization of InVO4 Nano-materials and their Photoluminescence Properties. Procedia Eng. 2014, 94, 64– 70,  DOI: 10.1016/j.proeng.2013.11.043
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    Synthesis and Characterization of InVO4 Nano-materials and their Photoluminescence Properties
    Shen, Jianchao; Yang, Hui; Shen, Qianhong; You, Zengyu
    Procedia Engineering (2014), 94 (), 64-70CODEN: PERNBE; ISSN:1877-7058. (Elsevier Ltd.)
    InVO4 nano-materials with various morphologies were prepd. via hydrothermal method. X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscope (SEM) were adopted to investigate the effects of various factors, such as pH value, V/In molar ratio, hydrothermal conditions and different surfactants, on the phase and morphol. of InVO4. Photoluminescence spectra were also employed to est. the photoluminescence properties of as-prepd. InVO4 samples. The results show that the pH value and V/In molar ratio have an important influence on the phase of InVO4, while the hydrothermal conditions and surfactants mainly affect the morphol. of InVO4. As-prepd. InVO4 samples display an intensive visible emission located in the green region with its max. intensity peak at 530 nm, and the morphol. and grain size have obvious effect on the fluorescence intensity of InVO4.
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    Zeng, G.-S.; Yu, J.; Zhu, H.-Y.; Liu, H.-L.; Xing, Q.-J.; Bao, S.-K.; He, S.; Zou, J.-P.; Au, C.-T. Controllable synthesis of InTaO4 catalysts of different morphologies using a versatile sol precursor for photocatalytic evolution of H2. RSC Adv. 2015, 5, 37603,  DOI: 10.1039/C5RA03638K
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    Controllable synthesis of InTaO4 catalysts of different morphologies using a versatile sol precursor for photocatalytic evolution of H2
    Zeng, Gui-Sheng; Yu, Jian; Zhu, Hong-Yun; Liu, Hui-Long; Xing, Qiu-Ju; Bao, Shao-Kui; He, Shun; Zou, Jian-Ping; Au, Chak-Tong
    RSC Advances (2015), 5 (47), 37603-37609CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    For the first time, InTaO4 photocatalysts with different morphologies and structures were facilely prepd. using a versatile sol precursor. SEM, TEM and HRTEM were used to characterize their structures and morphologies. The as-synthesized materials exhibit different physicochem. properties. Compared to InTaO4 nanoparticles and SiO2@InTaO4 core-shell nanospheres, InTaO4 nanofibers have higher light absorption ability, larger sp. surface area, and better photocatalytic activity for hydrogen evolution. We proposed mechanisms for the formation and photocatalytic activity of the three catalysts. The present work not only provides a new approach for the synthesis of tantalates that are different in morphol. and structure but also offers new insights into the controllable prepn. of photocatalysts through a versatile precursor for environmental and energy applications.
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    Journal of Chemical Physics (1989), 90 (7), 3443-57CODEN: JCPSA6; ISSN:0021-9606.
    The optical (4000-15,000 cm-1) spectra of the lanthanides doped into single crystal LaF3 were interpreted in terms of transitions within 4fN configurations. Energy matrixes combining free-ion terms with a crystal field for an approx. model which assumes C2v instead of the actual C2 site symmetry were diagonalized. Correlations were obtained between exptl. transition energies and the computed level structures. The results are reported of previously unpublished exptl. spectroscopic investigations of Nd3+ and Sm3+:LaF3, and predicted energy levels for Pm3+bLaF3. The spectroscopic data for each ion were independently interpreted using an effective-operator model, then the model parameters were intercompared. Systematic trends were identified, and a comprehensive energy level diagram is presented.
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    Gruber, J. B.; Nash, K. L.; Yow, R. M.; Sardar, D. K.; Valiev, U. V.; Uzokov, A. A.; Burdick, G. W. Spectroscopic and magnetic susceptibility analyses of the 7FJ and 5D4 energy levels of Tb3+(4f8) in TbAlO3. J. Lumin. 2008, 128, 1271– 1284,  DOI: 10.1016/j.jlumin.2007.12.041
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    Gruber, John B.; Nash, Kelly L.; Yow, Raylon M.; Sardar, Dhiraj K.; Valiev, Uygun V.; Uzokov, Abdulla A.; Burdick, Gary W.
    Journal of Luminescence (2008), 128 (8), 1271-1284CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
    Detailed analyses of spectroscopic and temp.-dependent magnetic susceptibility data are reported for the crystal-field split energy levels of the 7FJ and 5D4 of Tb3+ in stoichiometric single crystals of ortho-aluminate TbAlO3. The spectroscopic data include absorption spectra obtained between 2940-480 nm from 8-300 K. High resoln. fluorescence spectra are reported, representing transitions from 5D4 to 7F6,5,4, at a sample temp. of 85 K. Using crystal-field modeling techniques recently adapted for low symmetry systems, we have assigned all 58 exptl. Stark levels within the 7FJ and 5D4 manifolds, with a fitting std. deviation of 4.5 cm-1 (3.8 cm-1 rms error). As a further test, the theor. Stark levels and calcd. wavefunctions were used to det. the temp. dependence of the magnetic susceptibility along the c-axis of the TbAlO3 crystal. Good agreement is obtained between the calcd. susceptibility and temp.-dependent magnetic data reported earlier, including a prediction of a 0.2 cm-1 splitting of the ground-state quasi-doublet. The susceptibility calcn. also confirms the predicted ordering of states within the 7F6 multiplet manifold.
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    Cascales, C.; Zaldo, C. Spectroscopic Characterization and Systematic Crystal-Field Modeling of Optically Active Rare Earth RE3+ Ions in the Bismuth Germanate BiY1-xRxGeO5 Host. Chem. Mater. 2006, 18, 3742– 3753,  DOI: 10.1021/cm060785t
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    Spectroscopic Characterization and Systematic Crystal-Field Modeling of Optically Active Rare Earth R3+ Ions in the Bismuth Germanate BiY1-xRxGeO5 Host
    Cascales, C.; Zaldo, C.
    Chemistry of Materials (2006), 18 (16), 3742-3753CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The synthesis conditions and structural characteristics of polycryst. R-doped BiY1-xRxGeO5 or stoichiometric BiRGeO5 (R = trivalent rare earth Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, or Yb3+) germanate compds. are presented. The energy levels of each R3+ up to the absorption edge of the germanate matrix have been established from 10 K optical absorption and photoluminescence spectra measurements. Detailed single-electron C2(Cs) Hamiltonians combining free-ion and crystal-field interactions have been used to model the above 4fN configurations. Very satisfactory correlations were obtained between obsd. and the calcd. energy levels, and corresponding tabulated data are reported. Systematic trends in the evolution of crystal-field parameters have been identified. The proper estn. of phenomenol. crystal-field parameters and crystal-field strengths at the R3+ site for the whole Pr-Yb series is the first step in prospecting the possibilities of this germanate structure as a new solid-state laser host. The obsd. medium-high (∼660 cm-1) splitting for the 2F7/2 ground state of Yb3+ in BiYbGeO5 fulfills a prerequisite for an efficient Yb quasi-three-level laser operation. The possibility of accepting large amts. of optically active R3+ ions and the intense Raman shifts obsd. at high frequencies are other advantageous characteristics of the title host.
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    Porcher, P.; Couto Dos Santos, M.; Malta, O. Relationship between phenomenological crystal field parameters and the crystal structure: The simple overlap model. Phys. Chem. Chem. Phys. 1999, 1, 397– 405,  DOI: 10.1039/a803807d
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    Relationship between phenomenological crystal field parameters and the crystal structure: The simple overlap model
    Porcher, Pierre; Couto Dos Santos, Marcos; Malta, Oscar
    Physical Chemistry Chemical Physics (1999), 1 (3), 397-405CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The simple overlap model was used to calc. crystal field parameters from the crystal structure. It is applied to 25 rare earth and some actinide and 3d transition element compds. for which crystal field parameters were previously detd. exptl. The model is based on effective charges, located around the middle of the rare earth ion-ligand distance, which are assumed to be proportional to the magnitude of the overlap, ρ, between the rare earth ion and ligand wavefunctions. Good agreement between theor. and exptl. crystal field parameters is found for most of the cases studied. The best predictions were obtained when the overlap varies slightly from one rare earth compd. to another (0.05<ρ<0.08), in agreement with the general assumption that the f orbitals have a small part in the chem. bonding. For the two examples involving the 5f elements, the overlaps with the ligand have, curiously, the same magnitude as in the case of the rare earth elements. As expected, the 3d element wavefunctions show a stronger mixing with those of the ligands (0.10<ρ<0.25).
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    Hernández-Rodríguez, M. A.; Muñoz-Santiuste, J. E.; Lavín, V.; Lozano-Gorrín, A. D.; Rodríguez-Hernández, P.; Muñoz, A.; Venkatramu, V.; Martín, I. R.; Rodríguez-Mendoza, U. R. High pressure luminescence of Nd3+ in YAlO3 perovskite nanocrystals. A crystal-field analysis. J. Chem. Phys. 2018, 148, 044201  DOI: 10.1063/1.5010150
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    High pressure luminescence of Nd3+ in YAlO3 perovskite nanocrystals: A crystal-field analysis
    Hernandez-Rodriguez, Miguel A.; Munoz-Santiuste, Juan E.; Lavin, Victor; Lozano-Gorrin, Antonio D.; Rodriguez-Hernandez, Placida; Munoz, Alfonso; Venkatramu, Vemula; Martin, Inocencio R.; Rodriguez-Mendoza, Ulises R.
    Journal of Chemical Physics (2018), 148 (4), 044201/1-044201/10CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Pressure-induced energy blue- and red-shifts of the 4F3/2 → 4I9/2,11/2 near-IR emission lines of Nd3+ ions in YAlO3 perovskite nano-particles have been measured from ambient conditions up to 29 GPa. Different pos. and neg. linear pressure coeffs. have been calibrated for the emission lines and related to pressure-induced changes in the interactions between those Nd3+ ions and their twelve oxygen ligands at the yttrium site. Potentiality of the simple overlap model, combined with ab initio structural calcns., in the description of the effects of these interactions on the energy levels and luminescence properties of the optically active Nd3+ ion is emphasized. Simulations show how the energies of the 4f3 ground configuration and the barycenters of the multiplets increase with pressure, whereas the Coulomb interaction between f-electrons decreases and the crystal-field strength increases. All these effects combined explain the wavelength blue-shifts of some near-IR emission lines of Nd3+ ions. Large pressure rates of various emission lines suggest that a YAlO3 perovskite nano-crystal can be a potential candidate for near-IR optical pressure sensors. (c) 2018 American Institute of Physics.
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    Chang, N. C.; Gruber, J. B.; Leavitt, R. P.; Morrison, C. A. Optical spectra, energy levels, and crystal-field analysis of tripositive rare-earth ions in Y2O3. II. Kramers ions in C2 sites. J. Chem. Phys. 1982, 76, 3877,  DOI: 10.1063/1.443530
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    Optical spectra, energy levels, and crystal field analysis of tripositive rare earth ions in yttrium oxide. I. Kramers ions in C2 sites
    Chang, N. C.; Gruber, John B.; Leavitt, Richard P.; Morrison, Clyde A.
    Journal of Chemical Physics (1982), 76 (8), 3877-89CODEN: JCPSA6; ISSN:0021-9606.
    Optical absorption and fluorescence spectra of the Kramers ions Ce3+, Sm3+, Dy3+, and Yb3+ in the C2 sites of Y2O3 are reported. A crystal-field anal. of these data and previously reported data for Nd3+ and Er3+ was performed that is based on a Hamiltonian of C2 point-group symmetry, including J-mixing effects. A best-fit anal. was performed for the ions Nd3+, Sm3+, Dy3+, and Er3+; resulting in root-mean square deviations between calcd. and exptl. levels range from 5.1 to 7.9 cm-1. Results of this anal. were used to obtain a smoothed set of crystal-field parameters for the entire lanthanide series; these smoothed parameters were used to predict the energy levels of Ce3+, Yb3+, and Gd3+ in Y2O3. Predictions for Ce3+ and Yb3+ were compared with exptl. data.
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    Gluba, M. A.; Nickel, N. H.; Karpensky, N. Interstitial zinc clusters in zinc oxide. Phys. Rev. B 2013, 88, 245201,  DOI: 10.1103/PhysRevB.88.245201
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    Physical Review B: Condensed Matter and Materials Physics (2013), 88 (24), 245201/1-245201/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Doped zinc oxide (ZnO) exhibits anomalous Raman modes in the range of 270 to 870 cm-1. Commonly, the resonance at 275 cm-1 is attributed to the local vibration of Zn atoms in the vicinity of extrinsic dopants. We revisit this assignment by investigating the influence of isotopically purified zinc oxide thin films on the frequency of the vibrational mode around 275 cm-1. For this purpose, undoped and nitrogen-doped ZnO thin-films with Zn isotope compns. of natural Zn, 64Zn, 68Zn, and a 1:1 mixt. of 64Zn and 68Zn were grown by pulsed laser deposition. The isotopic shift and the line shape of the Raman resonance around 275 cm-1 are analyzed in terms of three different microscopic models, which involve the vibration of (i) interstitial zinc atoms bound to extrinsic defects, (ii) interstitial diat. Zn mols., and (iii) interstitial zinc clusters. The energy diagram of interstitial Zn-Zn bonds in a ZnO matrix is derived from d. functional theory calcns. The interstitial Zn-Zn bond is stabilized by transferring electrons from the antibonding orbital into the ZnO conduction band. This mechanism facilitates the formation of interstitial Zn clusters and fosters the common n-type doping asymmetry of ZnO.
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    Mondal, S.; Appalakondaiah, S.; Vaitheeswaran, G. High pressure structural, electronic, and optical properties of polymorphic InVO4 phases. J. Appl. Phys. 2016, 119, 085702  DOI: 10.1063/1.4942182
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    High pressure structural, electronic, and optical properties of polymorphic InVO4 phases
    Mondal, S.; Appalakondaiah, S.; Vaitheeswaran, G.
    Journal of Applied Physics (Melville, NY, United States) (2016), 119 (8), 085702/1-085702/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    In the present work, we report a detailed d. functional theory calcn. on polymorphic InVO4 phases by means of projector augmented wave method. The computed first-order structural phase transformation from orthorhombic (Cmcm) to monoclinic (P2/c) structure is found to occur around 5.6 GPa along with a large vol. collapse of 16.6%, which is consistent with previously reported exptl. data. This transformation also leads to an increase in the coordination no. of vanadium atom from 4 to 6. The computed equil. and high pressure structural properties of both InVO4 phases, including unit cell parameters, equation of state, and bulk moduli, are in good agreement with the available exptl. data. In addn., compressibility is found to be highly anisotropic and the b-axis being more compressible than the other for both the structures. Electronic band structures for both the phases were calcd., and the band gaps for orthorhombic and monoclinic InVO4 are found to be 4.02 and 1.67 eV, resp., within the Tran-Blaha Modified Becke-Johnson potential as implemented in linearized augmented planewave method. We further examd. the optical properties such as dielec. function, refractive index, and absorption spectra for both the structures. From the implications of these results, it can be proposed that the high pressure InVO4 phase can be more useful than orthorhombic phase for photo catalytic applications. (c) 2016 American Institute of Physics.
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    Zhou, J.; Huang, F.; Xu, J.; Chen, H.; Wang, Y. Luminescence study of a self-activated and rare earth activated Sr3La(VO4). J. Mater. Chem. C 2015, 3, 3023– 3028,  DOI: 10.1039/C4TC02783C
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    Luminescence study of a self-activated and rare earth activated Sr3La(VO4)3 phosphor potentially applicable in W-LEDs
    Zhou, Jiangcong; Huang, Feng; Xu, Ju; Chen, Hui; Wang, Yuansheng
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2015), 3 (13), 3023-3028CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    A novel Sr3La(VO4)3 phosphor is explored which exhibits the bright self-activated luminescence and the ability to sensitize emission of rare earth activators, under the excitation of near UV light. The self-activated emission of the Sr3La(VO4)3 host is peaked at 520 nm with a broad emission band ranging from 400 nm to 700 nm, ascribing to the charge transfer in (VO4)3- groups; while the Sr3La(VO4)3:Eu3+ phosphors show both the broad band luminescence of (VO4)3- groups and the sharp peak emissions of Eu3+ ions. Through doping concn. optimization, intense warm white light is achieved under near UV excitation, with a quantum yield of 31.2%. This phosphor is therefore potentially applicable in near UV chip excited white LEDs.
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    Ronde, H.; Blasse, G. The nature of the electronic transitions of the vanadate group. J. Inorg. Nucl. Chem. 1978, 40, 215– 219,  DOI: 10.1016/0022-1902(78)80113-4
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    Ronde, H.; Blasse, G.
    Journal of Inorganic and Nuclear Chemistry (1978), 40 (2), 215-19CODEN: JINCAO; ISSN:0022-1902.
    The luminescence of inorg. vanadates, e.g. NaCaVO4, Zn3(VO4)2, and LiMgP1-xVxO4 (x = 0.01, 0.05), was examd. and the obsd. emission assigned to the 3T1(t152e) → 1A1(t16) and 3T2(t152e) → 1A1 transitions. The 3T1(t152e) level is the lowest excited state of the VO4 group. The excitation was assigned to the 1A1(t16) → 1T1(t152e) and → 1T2(t152e) transitions. The position of the excitation bands is only slightly influenced by the host. The host-lattice dependence of the quenching temp. of the emission is discussed in terms of a configurational-coordinate model.
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    Nakajima, T.; Isobe, M.; Tsuchiya, T.; Ueda, Y.; Manabe, T. Correlation between Luminescence Quantum Efficiency and Structural Properties of Vanadate Phosphors with Chained, Dimerized, and Isolated VO4 Tetrahedra. J. Phys. Chem. C 2010, 114, 5160– 5167,  DOI: 10.1021/jp910884c
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    Correlation between Luminescence Quantum Efficiency and Structural Properties of Vanadate Phosphors with Chained, Dimerized, and Isolated VO4 Tetrahedra
    Nakajima, Tomohiko; Isobe, Masahiko; Tsuchiya, Tetsuo; Ueda, Yutaka; Manabe, Takaaki
    Journal of Physical Chemistry C (2010), 114 (11), 5160-5167CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The internal luminescence quantum efficiency and color properties of AVO3 (A = Li, Na, K, Rb, and Cs), M2V2O7 (M = Mg, Ca, Sr, Ba, and Zn), and M3V2O8 (M = Mg, Ca, Sr, Ba, and Zn) were studied. These vanadate phosphors exhibited broadband emission from 400 nm to over 800 nm due to the 1-electron charge transfer transition in the VO4 tetrahedra, and the color of the luminescent materials ranged from green to yellow-orange via white, corresponding to 0.277 < x < 0.494 and 0.389 < y < 0.488 on the CIE chromaticity diagram. The luminescence quantum efficiency of the vanadate phosphors with VO4 tetrahedra was strongly enhanced by the strong interaction between V ions and the weak interaction between V and A(M) ions in the crystal structures. The authors hypothesize that the long exciton diffusion lifetime induced by these structural features enhanced luminescence, leading to high quantum efficiency.
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    Van de Krol, R.; Ségalini, J.; Enache, C. S. Influence of point defects on the performance of InVO4 photoanodes. J. Photonics Energy 2011, 1, 016001  DOI: 10.1117/1.3564926
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    Influence of point defects on the performance of InVO4 photoanodes
    van de Krol, Roel; Segalini, Julie; Enache, Cristina S.
    Journal of Photonics for Energy (2011), 1 (), 016001/1-016001/10CODEN: JPEOBV; ISSN:1947-7988. (Society of Photo-Optical Instrumentation Engineers)
    The properties of thin film InVO4 photoanodes for water splitting have been studied. Compact films of InVO4 were prepd. by spray pyrolysis and are found to be stable between pH 3 and 11. Although the indirect bandgap is 3.2 eV, a modest amt. of visible light absorption is obsd. The origin of this absorption is attributed to the presence of deep donor states at ∼0.7 eV below the conduction band. These donor states presumably correspond to oxygen vacancies, which form as a result of small but unavoidable deviations of In:V from the ideal 1:1 stoichiometry during the wet-chem. synthesis process. Shallow donors are absent in this material, in contrast to what is normally obsd. for metal oxides. The deep donor model explains the much stronger visible light absorption of powders compared to thin films. The defect chem. reactions that lead to the formation of the deep donors are shown, and are supported by photoluminescence data.
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    Nakajima, T.; Isobe, M.; Tsuchiya, T.; Ueda, Y.; Manabe, T. Photoluminescence property of vanadates M2V2O7 (M: Ba, Sr and Ca). Opt. Mater. 2010, 32, 1618– 1621,  DOI: 10.1016/j.optmat.2010.05.021
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    Photoluminescence property of vanadates M2V2O7 (M: Ba, Sr and Ca)
    Nakajima, T.; Isobe, M.; Tsuchiya, T.; Ueda, Y.; Manabe, T.
    Optical Materials (Amsterdam, Netherlands) (2010), 32 (12), 1618-1621CODEN: OMATET; ISSN:0925-3467. (Elsevier B.V.)
    We investigated the photoluminescent properties including the internal quantum efficiency (η) for the M2V2O7 (M: Ba, Sr, and Ca) as a new vanadate phosphor system. In the M2V2O7, the broad band emission spectra from 400 to 800 nm derived from the charge transfer transition in the VO4 tetrahedra were obsd. The emission band of these compds. consisted of two maxima Em1 and Em2 corresponding to 3 T 1-1 A 1 and 3 T 2-1 A 1 transitions, resp. These two maxima red-shifted with the decreasing ionic radius of M-site, as a result, the luminescent color was varied from green (M: Ba) to yellowish orange (M: Ca). The η for the Ba2V2O7, Sr2V2O7 and Ca2V2O7 were 25%, 8% and 0.9%, resp. This enhancement of η in the M2V2O7 could be due to following structural features: the stronger V-V and weaker V-M correlations.
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    Strong quenching of Tb3+ emission by terbium-vanadium interaction in yttrium phosphate--yttrium vanadate
    DeLosh, Robert G.; Tien, T. Y.; Gibbons, E. F.; Zacmanidis, P. J.; Stadler, H. L.
    Journal of Chemical Physics (1970), 53 (2), 681-5CODEN: JCPSA6; ISSN:0021-9606.
    Solid solns. in the system YPO4-YVO4, with and without 2 mole % of Tb3+, were studied to det. the reason for the total inactivity of Tb in YVO4. The comparison of the diffuse reflectivity of Tb-contg. solns. with that of Tb-free ones shows a uv absorption at energies below the absorption edge of YVO4 in materials contg. both Tb and V. Selective excitation directly into Tb3+ in these wavelength regions produced reduced Tb3+ emission when there was V present. The band emission of the VO43+ groups, and the Tb3+ emission under electron beam excitation, were strongly quenched whenever Tb and V were both present. Emission of Dy3+, present as a 10-ppm impurity in all samples, was also quenched by the presence of both Tb and V, but not by either alone. All these observations imply energy loss by Tb-V interaction. A particular loss mechanism-the well-known intervalence transfer of charge from Tb to V resulting in an abrupt change of bond strength leading to a transfer of electronic excitation into lattice vibrations-is supported by measurements of Tb activity as a function of V concn. The intensity of Tb emission is proportionatel to the probability that none of 4 metal ion sites about a Tb is occupied by a V. This behavior suggests the intervalence transfer absorption mechanism, because only 4 of the nearest 6 metal ion sites have Tb-O-M bond angles (about 150°) consistent with good charge transfer from Tb to V; the remaining 2 sites have a Tb-O-M angle near 90° and should be relatively inactive in charge transfer.
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    Terbium ions-activated phosphors
    Blasse, G.; Bril, A.
    Philips Research Reports (1967), 22 (5), 481-504CODEN: PRREA9; ISSN:0031-7918.
    The fluorescence of many oxide, oxyhalide, and fluoride phosphors; activated by Tb(III), was studied. The dependence of the spectral energy distribution of the emission on the surroundings is much weaker than for Eu(III) activated phosphors. This may be caused by the absence of charge-transfer absorption bands in the uv. region for Tb(III)-activated oxides. The position of the 4f-5d absorption band depends strongly on the host lattice. Factors influencing the position of this band are discussed, esp. crystal-field splitting in the 5d levels. Energy transfer from a host lattice to the Tb(III) ion is less efficient than that to Eu(III). Several explanations of this phenomenon are given. The max. quantum efficiency in the uv region is detd. by deviation from inverse symmetry at the Tb(III) site and the position of the 4f-5d absorption band of the Tb(III) ion. The larger the deviation and the longer the wavelength of the 4f-5d band, the lower is the efficiency.
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    Huignard, A.; Gacoin, T.; Boilot, J.-P. Synthesis and Luminescence Properties of Colloidal YVO4:Eu Phosphors. Chem. Mater. 2000, 12, 1090– 1094,  DOI: 10.1021/cm990722t
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    Synthesis and Luminescence Properties of Colloidal YVO4:Eu Phosphors
    Huignard, Arnaud; Gacoin, Thierry; Boilot, Jean-Pierre
    Chemistry of Materials (2000), 12 (4), 1090-1094CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Concd. colloidal solns. of well-dispersed YVO4:Eu nanoparticles are synthesized by pptn. reactions at room temp. and stabilized by Na hexametaphosphate. x-ray diffraction and electron microscopy characterizations show that the cryst. nanoparticles exhibit an ellipsoidal form with 2 characteristic dimensions of ∼15 and 30 nm. In comparison with the bulk, a lower luminescence efficiency as well as a higher concn. quenching are obsd. These deviations are explained as the variations of some characteristics of the colloidal samples, such as the crystallinity and the surface chem. When these parameters are optimized, the quantum yield of the luminescence reaches 38% for the nanoparticles contg. a Eu concn. of 15%.
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    Energy transfer mechanisms in Yb3+ doped YVO4 near-infrared downconversion phosphor
    Wei, Xian-Tao; Huang, Shan; Chen, Yong-Hu; Guo, Chang-Xin; Yin, Min; Xu, Wu
    Journal of Applied Physics (2010), 107 (10), 103107/1-103107/5CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Upon UV light excitation, an intense near-IR (NIR) emission of Yb3+(2F5/2 → 2F7/2) around 980 nm is obsd. in YVO4:Yb3+ phosphors. Owing to host absorption of YVO4, a broad excitation band ranging from 250 to 350 nm is recorded when the Yb3+ emission was monitored, which suggests an efficient energy transfer from host to Yb3+ ions. The Yb3+ concn. dependence of the visible vanadate emission as well as the Yb3+ emission is investigated. The decay curve of vanadate emission is measured under the excitation of a 266 nm pulsed laser. The decay time of the vanadate emission at 500 nm is remarkably reduced by introducing Yb3+ ions, further verifying that the energy transfer from the vanadate host to the Yb3+ ions is very efficient. Cooperative energy transfer (CET) is discussed as a possible mechanism for the NIR emission. The YVO4:Yb3+ phosphor can convert each UV photon into two NIR photons via CET, which has potential application in the high efficiency silicon-based solar cells. (c) 2010 American Institute of Physics.
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    Li, G. L.; Yin, Z. Theoretical insight into the electronic, optical and photocatalytic properties of InMO4 (M = V, Nb, Ta) photocatalysts. Phys. Chem. Chem. Phys. 2011, 13, 2824– 2833,  DOI: 10.1039/B921143H
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    Theoretical insight into the electronic, optical and photocatalytic properties of InMO4 (M = V, Nb, Ta) photocatalysts
    Li, Guo-Ling; Yin, Zhen
    Physical Chemistry Chemical Physics (2011), 13 (7), 2824-2833CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The electronic and optical properties of InMO4 (M = V, Nb, Ta) photocatalysts are studied using first-principles calcns. For all InMO4, the calcd. band gaps are larger than the measured optical gaps, indicating the existence of sub-bandgap transitions. Impurity states and excitons are considered to interpret the characteristic absorption onsets in the measured UV-visible diffuse reflection spectra. The novel visible-light-active water-splitting photocatalytic properties of InMO4 are related to the sub-bandgap transitions. Correlation between the impurity states and the photocatalytic activities is discussed for InMO4via the conventional mechanism of photocatalytic water-splitting on semiconductors. An excitonic mechanism analogous to Photosystem II in plant photosynthesis is also proposed for the photocatalytic water-splitting process on InMO4.
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    Feng, H.; Hou, D.; Huang, Y.; Hu, X. Facile synthesis of porous InNbO4 nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties. J. Alloys Compd. 2014, 592, 301– 305,  DOI: 10.1016/j.jallcom.2013.12.261
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    Facile synthesis of porous InNbO4 nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties
    Feng, Huiyu; Hou, Dongfang; Huang, Yunhui; Hu, Xianluo
    Journal of Alloys and Compounds (2014), 592 (), 301-305CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    Porous InNbO4 nanofibers with diams. of 50-100 nm were prepd. by a facile electrospinning method combined with subsequent annealing. The resulting photocatalyst was characterized by x-ray diffraction (XRD), SEM, transmission electron microscopy (TEM), UV-visible diffuse reflectance spectra (UV-vis DRS), and nitrogen-sorption anal. The photocatalytic activity of the photocatalysts was evaluated by degrdn. of Rhodamine B under visible-light irradn. Results demonstrate that the photocatalytic activity of the as-formed porous InNbO4 nanofibers by electrospinning is improved, in comparison to that of the InNbO4 crystallites that were prepd. by a high-temp. solid-state reaction. The enhanced visible-light-driven photocatalytic activity is attributed to the porous nanofibrous architecture, high surface area, and narrower band gap.
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    Back, M.; Massari, A.; Boffelli, M.; Gonella, F.; Riello, P.; Cristofori, D.; Riccò, R.; Enrichi, F. Optical investigation of Tb3+-doped Y2O3 nanocrystals prepared by Pechini-type sol–gel process. J. Nanopart. Res. 2012, 14, 792,  DOI: 10.1007/s11051-012-0792-x
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    Optical investigation of Tb3+-doped Y2O3 nanocrystals prepared by Pechini-type sol-gel process
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    Journal of Nanoparticle Research (2012), 14 (4), 792/1-792/10CODEN: JNARFA; ISSN:1388-0764. (Springer)
    We report an optical study of Tb3+-doped Y2O3 nanocrystals synthesized by Pechini-type solgel method. The particles are investigated in terms of size and morphol. by means of X-ray diffraction and transmission electron microscopy anal. It is shown how the simple Pechini method allows for the growth of monocryst. nanoparticles with a vol.-weighted av. size of about 30 nm. The optical properties of Tb3+ in the host lattice are studied in terms of PL, PLE, and lifetimes. Moreover, a correlation between the type of decay curves, the emission and excitation bands' shapes, and the site location of the emitting Tb3+ in the host material Y2O3 is proposed.
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    Muenchausen, R. E.; Jacobsohn, L. G.; Bennett, B. L.; McKigney, E. A.; Smith, J. F.; Valdez, J. A.; Cooke, D. W. Effects of Tb doping on the photoluminescence of Y2O3:Tb nanophosphors. J. Lumin. 2007, 126, 838– 842,  DOI: 10.1016/j.jlumin.2006.12.004
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    Effects of Tb doping on the photoluminescence of Y2O3:Tb nanophosphors
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    Journal of Luminescence (2007), 126 (2), 838-842CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
    The effects of Tb doping on the photoluminescence (PL) of Y2O3:Tb nanophosphors were studied. Nanophosphors were prepd. by the glycine-nitrate soln. combustion technique using yttria and terbia powders as precursors. PL excitation spectra at room temp. consist of 2 overlapping bands centered at 277 and 304 nm, whereas emission spectra comprise several groups of lines corresponding to the 5D4→7FJ (J = 1-6) 4f electronic transitions of the Tb+3 ions. A direct comparison of nanophosphor and bulk concn.-quenching curves was obtained by annealing the nanophosphor powder and converting it to bulk material without altering the Tb concn. The peak in the nanophosphor concn.-quenching curve occurs at a concn. ∼3 times higher than that of the bulk.
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    Flores-Gonzalez, M. A.; Ledoux, G.; Roux, S.; Lebbou, K.; Perriat, P.; Tillement, O. Preparing nanometer scaled Tb-doped Y2O3 luminescent powders by the polyol method. J. Solid State Chem. 2005, 178, 989– 997,  DOI: 10.1016/j.jssc.2004.10.029
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    Preparing nanometer scaled Tb-doped Y2O3 luminescent powders by the polyol method
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    Journal of Solid State Chemistry (2005), 178 (4), 989-997CODEN: JSSCBI; ISSN:0022-4596. (Elsevier)
    Sub-micrometer Tb-doped Y2O3 luminescent powders were prepd. from nitrate precursors using the polyol method. Just after pptn., the powders consist of agglomerates with a spherical shape and a size ranging between 400 and 500 nm. Each agglomerate is composed of ultra-small crystallites (from 3 to 6 nm) of a bcc. oxide phase whose luminescence presents original features in comparison with bulk materials. Powders were further calcined at different temps. and for annealing <900°, highly cryst. samples with the classical green 5D4→7F5 luminescent transitions of Tb3+ ions are obtained. For optimized annealing temps., sintering between the agglomerates is avoided and a sub-micrometric powder with a narrow size distribution and a high luminescence is obtained.
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    Meng, Q.; Chen, B.; Xu, W.; Yang, Y.; Zhao, X.; Di, W.; Lu, S.; Wang, X.; Sun, J.; Cheng, L.; Yu, T.; Peng, Y. Size-dependent excitation spectra and energy transfer in Tb3+-doped Y2O3 nanocrystalline. J. Appl. Phys. 2007, 102, 093505– 093501,  DOI: 10.1063/1.2803502
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    Size-dependent excitation spectra and energy transfer in Tb3+-doped Y2O3 nanocrystalline
    Meng, Qingyu; Chen, Baojiu; Xu, Wu; Yang, Yanmin; Zhao, Xiaoxia; Di, Weihua; Lu, Shaozhe; Wang, Xiaojun; Sun, Jiashi; Cheng, Lihong; Yu, Tao; Peng, Yong
    Journal of Applied Physics (2007), 102 (9), 093505/1-093505/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Nanocrystal Y2O3 powders with different grain sizes and various doping concns. of Tb3+ were prepd. by an autocombustion reaction. The size and surface effects on the 4f-5d transitions and energy transfers between Tb3+ ions were studied by using x-ray diffraction, TEM, fluorescent spectra, and luminescent decay. The excitation spectra are composed of 2 parts: 1 is the contribution from the Tb3+ at/near the nanoparticle surfaces; another is from the Tb3+ inside the nanoparticles. The study on the concn. quenching and luminescent decay indicated that the energy transfers depopulating the 5D3 and 5D4 level were assigned to the mechanisms of elec. dipole-dipole and exchange interaction, resp. The size confinement greatly affects the energy transfer quenching the emission from the 5D3 level, but slightly affects the energy transfer quenching the emission from the 5D4 level.
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    Pavitra, E.; Raju, G. S. R.; Ko, Y. H.; Yu, J. S. A novel strategy for controllable emissions from Eu3+ or Sm3+ ions co-doped SrY2O4:Tb3+ phosphors. Phys. Chem. Chem. Phys. 2012, 14, 11296– 11307,  DOI: 10.1039/c2cp41722g
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    A novel strategy for controllable emissions from Eu3+ or Sm3+ ions co-doped SrY2O4:Tb3+ phosphors
    Pavitra, E.; Raju, G. Seeta Rama; Ko, Yeong Hwan; Yu, Jae Su
    Physical Chemistry Chemical Physics (2012), 14 (32), 11296-11307CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Trivalent rare-earth (RE) ions (Eu3+, Tb3+ and Sm3+) activated multicolor emitting SrY2O4 phosphors were synthesized by a sol-gel process. The structural and morphol. studies were performed by the measurements of X-ray diffraction profiles and scanning electron microscope (SEM) images. The pure phase of SrY2O4 appeared after annealing at 1300 °C and the doping of RE ions did not show any effect on the structural properties. From the SEM images, the closely packed particles were obsd. due to the roughness of each particle tip. The photoluminescence (PL) anal. of individual RE ions activated SrY2O4 phosphors exhibits excellent emission properties in their resp. regions. The Eu3+ co-activated SrY2O4:Tb3+ phosphor creates different emissions by controlling the energy transfer from Tb3+ to Eu3+ ions. Based on the excitation wavelengths, multiple (green, orange and white) emissions were obtained by Sm3+ ions co-activated with SrY2O4:Tb3+ phosphors. The decay measurements were carried out for analyzing the energy transfer efficiency and the possible ways of energy transfer from donor to acceptor. The cathodoluminescence properties of these phosphors show similar behavior as PL properties except the energy transfer process. The obtained results indicated that the energy transfer process was quite opposite to the PL properties. The calcd. CIE chromaticity coordinates of RE ions activated SrY2O4 phosphors confirmed the red, green, orange and white emissions.
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    Chukova, O.; Nedilko, S.; Titov, Y.; Sheludko, V. Crystallographic Features and Nature of Luminescence Centres of the Niobate and Tantalate Compounds with Layered Perovskite-Like Structure. Open Mater. Sci. J. 2018, 12, 2– 13,  DOI: 10.2174/1874088X01812010002
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    Cryslallographic features and nature of luminescence centres of the niobate and tantalate compounds with layered perovskite-like structure
    Chukova, Oksana; Nedilko, Sergiy; Titov, Yuriy; Sheludko, Vadym
    Open Materials Science Journal (2018), 12 (), 2-13CODEN: OMSJAH; ISSN:1874-088X. (Bentham Open)
    Aim and Objectives The ultrafine powders of the perovskite-like layered AII3LaM3O12 (AII = Sr, Ba; M = Nb, Ta) compds. have been synthesized by heat treatment of co-pptd. hydroxy-carbonates. The luminescence of these compds. is reported for the first time. Methods: Luminescence spectra of all studied compds. are complex and contain two main wide bands with maxima near 2.9 and 2.5 eV. These luminescence bands were assigned to radiation electron transitions in the MoO67- mol. groups of different symmetry located in the various lattice positions. Conclusion: The energy levels scheme of the MoO67- group and related radiation and absorption transitions had been proposed.
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    Van Pieterson, L.; Heeroma, M.; de Heer, E.; Meijerink, A. Charge transfer luminescence of Yb3+. J. Lumin. 2000, 91, 177– 193,  DOI: 10.1016/S0022-2313(00)00214-3
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    Journal of Luminescence (2000), 91 (3&4), 177-193CODEN: JLUMA8; ISSN:0022-2313. (Elsevier Science B.V.)
    A systematic study of charge transfer (CT) luminescence from Yb3+ is presented. CT luminescence was obsd. in LiYF4 (≈7 eV), aluminates, phosphates, oxides (≈ 4 eV) and oxysulfides (≈ 3 eV). In all cases the CT emission bands are broad and the Stokes shifts are large, ranging from 7000 cm-1 in oxysulfides and LiYF4 and up to 17,000 cm-1 in aluminates. The influence of the size of the host lattice cation site on the charge transfer luminescence was studied. A shift to longer wavelengths of the CT emission bands with increasing size of the cation site was obsd. Typical (radiative) decay times for the CT luminescence are 100 - 200 ns. The quenching of the Yb3+ CT luminescence in the various host lattices was studied and compared with the quenching of the Eu3+ emission under CT excitation in the same lattices. A good correlation was found. The quenching temps. of the Eu3+ emission are much higher than that of the Yb3+ CT luminescence. In lattices where Eu3+ emission quenches at very high temps. (= 500K) Yb3+ CT luminescence could be obsd.
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    Why so deep research on Yb3+-doped optical inorganic materials?
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    A review. The state-of-art of Yb3+-doped inorg. materials will be presented due to strong interests in basics research and applied research. 4f13 configuration will be illustrated by few examples such as optical transition assignments in the IR range, diode pumped-IR laser, figure-of-merit of laser crystals, concn. quenching studies, evidence of Yb3+ pairs, the advance of Yb3+-doped cubic ceramics for laser sources, Yb3+ as sensitizer of activator ions and Yb3+ ion as structural probes of hosts. The last results analyzed will concern charge transfer processes of Yb3+ in oxides and the interests in luminescence.
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    Host lattice dependences of luminescence spectra and relaxation properties of Tb3+ are described. Reduced matrix elements relevant to the transitions were calcd. and compared with observations. The spectra and the decay times of the 5D4 → 7FJ transitions are in qual. agreement with the calcns. The elec.-dipole transition of 5D4 → 7F5 has the largest probability in the 5D4 → 7FJ emission transitions within the framework of the Judd-Ofelt approxn. When the crystal field acting on Tb3+ is very strong, the intensity of 5D4 → 7F3 can become comparable to that of 5D4 → 7F5. Magnetic-dipole transitions always have the largest probability for 5D4 → 7F5. Spontaneous multiphonon relaxation of 5D3 → 5D4 is sometimes a dominant decay mechanism of the 5D3 state. The obsd. multiphonon relaxation rates are ∼2 × 102 in YPO4, ∼8 × 103 in YBO3 and ∼2 × 104 in ScBO3 groups in units of sec-1.
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    Tb3+ Luminescence in Tb-Doped and Tb/Gd-Doped CsCdBr3 Crystals: 5D4→5D3 Cross-Relaxation Rates in Tb3+ Pairs
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    The luminescence properties of Tb3+ in Tb3+-Tb3+ and Tb3+-Gd3+ majority-type pairs in CsCdBr3 are studied at 77 K and room temp. 5D3→5D4 cross-relaxation rates for Tb3+ in Tb3+-Tb3+ pairs are detd. The intrinsic 5D3→5D4 and 5D3→7FJ relaxation rates of Tb3+ in the majority-pair site are estd. The relative nos. of Tb3+-Tb3+ and Tb3+-Gd3+ pairs in Tb/Gd-doped CsCdBr3 are consistent with statistical pair formation.
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    The luminescence efficiency of the alkali metavanadates has been reported to increase by several orders of magnitude in the cation series from Na to K to Rb to Cs. Measurements of luminescence efficiency and decay time as a funcion of temp. show that max. occur in the luminescence efficiency at 300, 200, and 170°K and that the decay times for the predominantly exponential decay rise from between 10 and 20 μsec at room temp. at 160,300, and >1500 μsec at 77°K for KVO3, RbVO3, and CsVO3, resp. These results are interpreted in terms of thermally activated exciton diffusion from the absorbing site to recombination centers along chains of (VO4) tetrahedral complexes which occur in the metavanadate structure. The lifetime of excitons and the activation energy for exciton diffusion is related to the degree of polarization of the anionic complex induced by the cation.
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    Laversenne, L.; Guyot, Y.; Goutaudier, C.; Cohen-Adad, M. T.; Boulon, G. Optimization of spectroscopic properties of Yb3+-doped refractory sesquioxides: cubic Y2O3, Lu2O3 and monoclinic Gd2O3. Opt. Mater. 2001, 16, 475– 483,  DOI: 10.1016/S0925-3467(00)00095-1
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    Optimization of spectroscopic properties of Yb3+-doped refractory sesquioxides: cubic Y2O3, Lu2O3 and monoclinic Gd2O3
    Laversenne, L.; Guyot, Y.; Goutaudier, C.; Cohen-Adad, M. T.; Boulon, G.
    Optical Materials (Amsterdam, Netherlands) (2001), 16 (4), 475-483CODEN: OMATET; ISSN:0925-3467. (Elsevier Science B.V.)
    Absorption and emission spectra are given for Yb3+-doped Y2O3, Lu2O3 and Gd2O3 at room temp. Y2O3 and Lu2O3 as close cubic matrixes, show Yb3+ similar spectra different of Yb3+ in Gd2O3 monoclinic structure. Here, the authors use a new method to study and optimize the main spectroscopic properties with only one concn. gradient sample. Finally, assignments of Yb3+ Stark levels and Raman vibrations in Y2O3, Lu2O3 and Gd2O3 single crystal are given.
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    Bharat, L. K.; Jeon, S. K.; Krishna, K. G.; Yu, J. S. Rare-earth free self-luminescent Ca2KZn2(VO4)3 phosphors for intense white light-emitting diodes. Sci. Rep. 2017, 7, 42348,  DOI: 10.1038/srep42348
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    Rare-earth free self-luminescent Ca2KZn2(VO4)3 phosphors for intense white light-emitting diodes
    Bharat, L. Krishna; Jeon, Soo-Kun; Krishna, Kurugundla Gopi; Yu, Jae Su
    Scientific Reports (2017), 7 (), 42348CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    The com. available white-light-emitting diodes (WLEDs) are made with a combination of blue LEDs and yellow phosphors. These types of WLEDs lack certain properties which make them meagerly applicable for general illumination and flat panel displays. The soln. for such problem is to use near-UV (NUV) chips as an excitation source because of their high excitation efficiency and good spectral distribution. Therefore, there is an active search for new phosphor materials which can be effectively excited within the NUV wavelength range (350-420 nm). In this work, novel rare-earth free self-luminescent Ca2KZn2(VO4)3 phosphors were synthesized by a citrate assisted sol-gel method at low calcination temps. Optical properties, internal quantum efficiency and thermal stability as well as morphol. and crystal structure of Ca2KZn2(VO4)3 phosphors for their application to NUV-based WLEDs were studied. The crystal structure and phase formation were confirmed with XRD patterns and Rietveld refinement. The optical properties of these phosphor materials which can change the NUV excitation into visible yellow-green emissions were studied. The synthesized phosphors were then coated onto the surface of a NUV chip along with a blue phosphor (LiCaPO4:Eu2+) to get brighter WLEDs with a color rendering index of 94.8 and a correlated color temp. of 8549 K.
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  • Abstract

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    Figure 1

    Figure 1. Crystal structure of the wolframite-type InNb(Ta)O4 host and coordination environments for Nb5+(Ta5+) and In3+ (left) and orthorhombic InVO4 host and coordination environments for V5+ and In3+ (right) (for the color code of the structure, the reader is referred to the digital version).

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    Figure 2

    Figure 2. XRD patterns of InVO4, InNbO4, and InTaO4 doped samples with Tb3+ or Yb3+. Bars/columns data represent the standard ICSD charts of the undoped orthorhombic InVO4 (ICSD-237482) and the undoped monoclinic InNbO4 (ICSD-257869) and InTaO4 (ICSD-72569), respectively. The height of the bars is proportional to the theoretical intensity of the peaks. Tables indicating the index, positions, and intensities of all reflections are included in the Supporting Information.

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    Figure 3

    Figure 3. RS spectra of InVO4, InNbO4, and InTaO4 compounds and the corresponding doped samples with Tb3+ or Yb3+.

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    Figure 4

    Figure 4. Optical reflectance (top) and PLE/PL spectra (bottom) of InVO4, InVO4:Tb, and InVO4:Yb (short dashed-dotted line corresponds to PLE data). The data were normalized for a better comparison of the emitted signals.

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    Figure 5

    Figure 5. Energy level diagram of InVO4 doped with Yb (top) and InNbO4 doped with Yb or Tb (ET stands for electron transfer. CR stands for cross-relaxation. Dashed arrows represent nonradiative processes, and solid arrows correspond to PL emission and excitation).

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    Figure 6

    Figure 6. Optical reflectance (top) and PLE/PL spectra (bottom) of InNbO4, InNbO4:Tb, and InNbO4:Yb (short dasheddotted line corresponds to PLE data). The data were normalized for a better comparison of the emitted signals.

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    Figure 7

    Figure 7. Optical reflectance (top) and PLE/PL spectra (bottom) of InTaO4, InTaO4:Tb, and InTaO4:Yb (short dasheddotted line corresponds to PLE data). The data were normalized for a better comparison of the emitted signals.

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    Figure 8

    Figure 8. Time-resolved PL decay in the visible and NIR spectral regions for each matrix. The excitation wavelength was 310 nm, and emission wavelengths were 550 nm for visible and 995 nm for NIR.

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    Figure 9

    Figure 9. CIE diagram of InVO4, InNbO4:Tb, and InTaO4:Tb.

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  • References

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    This article references 69 other publications.

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      A pressure-induced phase transition, assocd. with an increase of the coordination no. of In and Ta, is detected beyond 13 GPa in InTaO4 by combining synchrotron x-ray diffraction and Raman measurements in a diamond-anvil cell with ab initio calcns. High-pressure optical-absorption measurements were also carried out. The high-pressure phase has a monoclinic structure that shares the same space group with the low-pressure phase (P2/c). The structure of the high-pressure phase can be considered as a slight distortion of an orthorhombic structure described by space group Pcna. The phase transition occurs together with a unit-cell vol. collapse and an electronic band-gap collapse obsd. by expts. and calcns. Addnl., a band crossing is found to occur in the low-pressure phase near 7 GPa. The pressure dependence of all the Raman-active modes is reported for both phases as well as the pressure dependence of unit-cell parameters and the equations of state. Calcns. also provide information on IR-active phonons and bond distances. These findings provide insights into the effects of pressure on the phys. properties of InTaO4.
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    10. 10
      Botella, P.; Errandonea, D.; Garg, A. B.; Rodríguez-Hernandez, P.; Muñoz, A.; Achary, S. N.; Vomiero, A. High-pressure characterization of the optical and electronic properties of InVO4, InNbO4, and InTaO4. SN Appl. Sci. 2019, 1, 389,  DOI: 10.1007/s42452-019-0406-7
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      Garg, A. B.; Errandonea, D.; Popescu, C.; Martinez-García, D.; Pellicer-Porres, J.; Rodríguez-Hernández, P.; Muñoz, A.; Botella, P.; Cuenca-Gotor, V. P.; Sans, J. A. Pressure-Driven Isostructural Phase Transition in InNbO4: In Situ Experimental and Theoretical Investigations. Inorg. Chem. 2017, 56, 5420,  DOI: 10.1021/acs.inorgchem.7b00437
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      11
      Pressure-Driven Isostructural Phase Transition in InNbO4: In Situ Experimental and Theoretical Investigations
      Garg, Alka B.; Errandonea, Daniel; Popescu, Catalin; Martinez-Garcia, Domingo; Pellicer-Porres, Julio; Rodriguez-Hernandez, Placida; Munoz, Alfonso; Botella, Pablo; Cuenca-Gotor, Vanesa P.; Sans, Juan Angel
      Inorganic Chemistry (2017), 56 (9), 5420-5430CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The high-pressure behavior of technol. important visible-light photocatalytic semiconductor InNbO4, adopting a monoclinic wolframite-type structure at ambient conditions, was investigated using synchrotron-based X-ray diffraction, Raman spectroscopic measurements, and first-principles calcns. The exptl. results indicate the occurrence of a pressure-induced isostructural phase transition in the studied compd. beyond 10.8 GPa. The large vol. collapse assocd. with the phase transition and the coexistence of two phases obsd. over a wide range of pressure shows the nature of transition to be first-order. There is an increase in the oxygen anion coordination no. around In and Nb cations from six to eight at the phase transition. The ambient-pressure phase has been recovered on pressure release. The exptl. pressure-vol. data when fitted to a Birch-Murnaghan equation of states yields the value of ambient pressure bulk modulus as 179(2) and 231(4) GPa for the low- and high-pressure phases, resp. The pressure dependence of the Raman mode frequencies and Gruneisen parameters was detd. for both phases by exptl. and theor. methods. The same information is obtained for the IR modes from first-principles calcns. Results from theor. calcns. corroborate the exptl. findings. They also provide information on the compressibility of interat. bonds, which is correlated with the macroscopic properties of InNbO4.
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    12. 12
      Errandonea, D.; Gomis, O.; García-Domene, B.; Pellicer-Porres, J.; Katari, V.; Achary, S. N.; Tyagi, A. K.; Popescu, C. New Polymorph of InVO4: A High-Pressure Structure with Six-Coordinated Vanadium. Inorg. Chem. 2013, 52, 12790,  DOI: 10.1021/ic402043x
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      12
      New Polymorph of InVO4: A High-Pressure Structure with Six-Coordinated Vanadium
      Errandonea, Daniel; Gomis, Oscar; Garcia-Domene, Braulio; Pellicer-Porres, Julio; Katari, Vasundhara; Achary, S. Nagabhusan; Tyagi, Avesh K.; Popescu, Catalin
      Inorganic Chemistry (2013), 52 (21), 12790-12798CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A new wolframite-type polymorph of InVO4 is identified under compression near 7 GPa by in situ high-pressure (HP) X-ray diffraction (XRD) and Raman spectroscopic investigations on the stable orthorhombic InVO4. The structural transition is accompanied by a large vol. collapse (ΔV/V = -14%) and a drastic increase in bulk modulus (from 69 to 168 GPa). Both techniques also show the existence of a third phase coexisting with the low- and high-pressure phases in a limited pressure range close to the transition pressure. XRD studies revealed a highly anisotropic compression in orthorhombic InVO4. In addn., the compressibility becomes nonlinear in the HP polymorph. The vol. collapse in the lattice is related to an increase of the polyhedral coordination around the vanadium atoms. The transformation is not fully reversible. The drastic change in the polyhedral arrangement obsd. at the transition is indicative of a reconstructive phase transformation. The HP phase here found is the only modification of InVO4 reported to date with 6-fold coordinated vanadium atoms. Finally, Raman frequencies and pressure coeffs. in the low- and high-pressure phases of InVO4 are reported.
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    13. 13
      Errandonea, D.; Tu, C.; Jia, G.; Martín, I. R.; Rodríguez-Mendoza, U. R.; Lahoz, F.; Torres, M. E.; Lavín, V. Effect of pressure on the luminescence properties of Nd3+ doped SrWO4 laser crystal. J. Alloys Compd. 2008, 451, 212– 214,  DOI: 10.1016/j.jallcom.2007.04.180
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      Effect of pressure on the luminescence properties of Nd3+ doped SrWO4 laser crystal
      Errandonea, D.; Tu, Chaoyang; Jia, Guohua; Martin, I. R.; Rodriguez-Mendoza, U. R.; Lahoz, F.; Torres, M. E.; Lavin, V.
      Journal of Alloys and Compounds (2008), 451 (1-2), 212-214CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      The luminescence spectra of the 4F3/2→4I9/2 transition of Nd3+ ions in a SrWO4 crystal were analyzed as a function of pressure at room temp. Expts. were performed in a diamond-anvil cell up to 13 GPa. At ∼10 GPa some changes in the emission spectra were obsd. which are attributed to a structural phase transition of the SrWO4 matrix. These results are in good agreement with a previous paper, in which in a pure SrWO4 matrix a scheelite to fergusonite phase transition is found around 10.5 GPa. Also, with increasing pressure, the decay curves from the 4F3/2 are nonexponential and faster indicating that the energy transfer processes between Nd3+ ions are important.
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    14. 14
      Muñoz-Santiuste, J. E.; Lavín, V.; Rodríguez-Mendoza, U. R.; Ferrer-Roca, C.; Errandonea, D.; Martínez-García, D.; Rodríguez-Hernández, P.; Muñoz, A.; Bettinelli, M. Experimental and theoretical study on the optical properties of LaVO4 crystals under pressure. Phys. Chem. Chem. Phys. 2018, 20, 27314,  DOI: 10.1039/C8CP04701D
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      14
      Experimental and theoretical study on the optical properties of LaVO4 crystals under pressure
      Munoz-Santiuste, J. E.; Lavin, V.; Rodriguez-Mendoza, U. R.; Ferrer-Roca, Ch.; Errandonea, D.; Martinez-Garcia, D.; Rodriguez-Hernandez, P.; Munoz, A.; Bettinelli, M.
      Physical Chemistry Chemical Physics (2018), 20 (43), 27314-27328CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We report optical absorption and luminescence measurements in pure and trivalent neodymium (Nd3+) doped LaVO4 crystals up to 25 GPa. Nd3+ luminescence has been employed as a tool to follow the structural changes in the crystal. We also present band-structure and crystal-field calcns. that provide the theor. framework to accurately explain the obsd. exptl. results. In particular, both optical absorption and luminescence measurements evidence that a phase transition takes place close to 12 GPa. They also provide information on the pressure dependence of the band-gap as well as the emission lines under compression. We found drastic changes in the optical properties of LaVO4 when the phase transition to a BaWO4-II structure occurs, which can be related to changes in the coordination no. of vanadium ions and in the local sites of Nd3+. Reported results are analyzed in comparison with those of previous X-ray diffraction and Raman expts., as well as with the features of related compds. For the first time, a consistent picture is reported explaining the behavior of the optical and electronic properties of LaVO4 at high-pressures.
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    15. 15
      Yoo, S. H.; Kum, J. M.; Cho, S. O. Tuning the electronic band structure of PCBM by electron irradiation. Nanoscale Res. Lett. 2011, 6, 545,  DOI: 10.1186/1556-276X-6-545
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      Tuning the electronic band structure of PCBM by electron irradiation
      Yoo, Seung Hwa; Kum, Jong Min; Cho, Sung Oh
      Nanoscale Research Letters (2011), 6 (1), 545/1-545/7CODEN: NRLAAD; ISSN:1556-276X. (Springer)
      Tuning the electronic band structures such as band-edge position and bandgap of org. semiconductors is crucial to maximize the performance of org. photovoltaic devices. We present a simple yet effective electron irradn. approach to tune the band structure of [6, 6]-phenyl-C61-butyric acid Me ester (PCBM) that is the most widely used org. acceptor material. We have found that the LUMO (LUMO) level of PCBM up-shifts toward the vacuum energy level, while the HOMO (HOMO) level down-shifts when PCBM is electron-irradiated. The shift of the HOMO and the LUMO levels increases as the irradiated electron fluence increases. Accordingly, the band-edge position and the bandgap of PCBM can be controlled by adjusting the electron fluence. Characterization of electron-irradiated PCBM reveals that the variation of the band structure is attributed to the mol. structural change of PCBM by electron irradn.
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    16. 16
      Tapasztó, L.; Dobrik, G.; Nemes-Incze, P.; Vertesy, G.; Lambin, P.; Biró, L. P. Tuning the electronic structure of graphene by ion irradiation. Phys. Rev. B 2008, 78, 233407,  DOI: 10.1103/PhysRevB.78.233407
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      16
      Tuning the electronic structure of graphene by ion irradiation
      Tapaszto, L.; Dobrik, G.; Nemes-Incze, P.; Vertesy, G.; Lambin, Ph.; Biro, L. P.
      Physical Review B: Condensed Matter and Materials Physics (2008), 78 (23), 233407/1-233407/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Mech. exfoliated graphene layers deposited on SiO2 substrate were irradiated with Ar+ ions in order to exptl. study the effect of at. scale defects and disorder on the low-energy electronic structure of graphene. The irradiated samples were investigated by scanning tunneling microscopy and spectroscopy measurements, which reveal that defect sites, besides acting as scattering centers for electrons through local modification of the on-site potential, also induce disorder in the hopping amplitudes. The most important consequence of the induced disorder is the substantial redn. in the Fermi velocity, revealed by bias-dependent imaging of electron-d. oscillations obsd. near defect sites.
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    17. 17
      Shih, H.-R.; Liu, K.-T.; Teoh, L.-G.; Wei, L.-K.; Chang, Y.-S. Synthesis and photoluminescence properties of (La,Pr) co-doped InVO4 phosphor. Microelectron. Eng. 2015, 148, 10– 13,  DOI: 10.1016/j.mee.2015.07.007
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      17
      Synthesis and photoluminescence properties of (La, Pr) co-doped InVO4 phosphor
      Shih, Hung-Rung; Liu, Kuan-Ting; Teoh, Lay-Gaik; Wei, Li-Kai; Chang, Yee-Shin
      Microelectronic Engineering (2015), 148 (), 10-13CODEN: MIENEF; ISSN:0167-9317. (Elsevier B.V.)
      (La3 +, Pr3 +) co-doped InVO4 phosphors were prepd. using a sol-gel method. For the (In1 - xPrx)VO4 (x = 0-0.1) system, X-ray powder diffraction (XRD) patterns show that all of the peaks are attributed to the orthorhombic phase, and the SEM (SEM) images show that there is no obvious difference for the particle morphol. between various Pr3 + ions doping. No emission peak characteristics of Pr3 + ions were obsd. under an excitation of 326 nm. In the (In0.97 - yLayPr0.03)VO4 (y = 0-0.97) system, XRD patterns indicated that the crystal structure changed from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure when the In3 + ions were gradually substituted by the La3 + ions. The location of excitation peaks shifted to short wavelength area (blue shift), and the characteristic f-f transition of Pr3 + ion was obsd. With increasing La3 + ion concn., a series of emission peaks assigned to the 3P0 → 3H4,5,6, 1D2 → 3H4, and 3P0 → 3F2 characteristic transitions of Pr3 + ion were obsd. in the emission spectra under an excitation of 315 nm, and this indicated that the La3 + ion can act as a sensitizer for the (In0.97 - yLayPr0.03)VO4 phosphor. The Commission Internationale de I'Eclairage (CIE) color chromaticity coordinates of (In0.97 - yLayPr0.03)VO4 phosphors vary with the La3 + ion concn. from the yellowish light region, though the green-yellow and finally to the near-white light area as the La3 + ion concn. increased.
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    18. 18
      Lu, M.; Li, Q.; Zhou, C.; Zhang, C.; Shi, H. Effects of nonmetal elements doping on the electronic structures of InNbO4: first-principles calculations. Mater. Res. Express 2018, 5, 075505  DOI: 10.1088/2053-1591/aace04
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      18
      Effects of nonmetal elements doping on the electronic structures of InNbO4: first-principles calculations
      Lu, Mengfei; Li, Qiaoqiao; Zhou, Changping; Zhang, Chengliang; Shi, Haifeng
      Materials Research Express (2018), 5 (7), 075505/1-075505/8CODEN: MREAC3; ISSN:2053-1591. (IOP Publishing Ltd.)
      To explore the effects of nonmetal elements mono-doping, the electronic structures of (B, C, P, F) doped InNbO4 have been investigated systematically based on the first-principles calcns. The obtained results revealed that the band edges of (B, C, P) doped InNbO4 slightly expanded while the band gap of F doped InNbO4 barely changed. Meanwhile, different p orbital states were introduced into the band gaps of InNbO4 after (B, C, P) doping, which was in favor of extending the absorption spectra to the visible-light region. Among these nonmetal doping systems, it was noted that the band edge potentials of InNbO4 satisfied the requirement for water splitting in C doped InNbO4. The localized C 2p states could perform as intermediate levels to reduce the transition energy of photo-induced electrons. Therefore, we put forward that C atom should be a suitable dopant for single anion doping InNbO4, which will provide a guideline for developing doping InNbO4 with visible-light sensitive reactions.
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    19. 19
      Song, Y.; Sun, Z.; Wu, Y.; Chai, Z.; Wang, X. Investigation of the Preferential Doping Site and Regulating on the Visible Light Response and Redox Performance for Fe- and/or La Doped InNbO4. Inorg. Chem. 2018, 57, 8558– 8567,  DOI: 10.1021/acs.inorgchem.8b01287
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      19
      Investigation of the Preferential Doping Site and Regulating on the Visible Light Response and Redox Performance for Fe- and/or La-Doped InNbO4
      Song, Yanyong; Sun, Zhebin; Wu, Yuhang; Chai, Zhanli; Wang, Xiaojing
      Inorganic Chemistry (2018), 57 (14), 8558-8567CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The preferential doping site, visible light response, and redox potential of Fe- and/or La-doped InNbO4 (INO) were studied using 1st-principles d. functional theory. Eight designed doping models, including Fe and/or La doping at In or/and Nb sites of INO are constructed, resp. It was found that Fe-doping and Fe,La-codoping to substitute In into an INO cell are energetically favorable, confirming that the steric hindrance plays a vital role for the selective doping site than the charge of the dopants. Fe doping always formed 2 impurity bands between the conduction and valence bands, originated from Fe 3d state, inferring the well visible light response. The presence of La has a specific regulation effects for Fe doping although the energy levels of the single La-doped models were completely similar to those of the undoped INO. The electron exchange between La and Fe dopants results in the significant interaction for codoping INO. Importantly, by doping La into INO cell, the redox potentials of Fe-doped INO could be well-regulated. The band potential moved to the more pos. energy level of the models Fe-doped at Nb sites, while it shifted to the more neg. level if Fe was doped at In site of La-INO. This study may provide the guidance for the designative dopants to construct the photocatalyst with stable, visible response, and good redox performance.
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    20. 20
      Rakesh, K.; Khaire, S.; Bhange, D.; Dhanasekaran, P.; Deshpande, S. S.; Awate, S. V.; Gupta, N. M. Role of doping-induced photochemical and microstructural properties in the photocatalytic activity of InVO4 for splitting of water. J. Mater. Sci. 2011, 46, 5466– 5476,  DOI: 10.1007/s10853-011-5489-5
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      20
      Role of doping-induced photochemical and microstructural properties in the photocatalytic activity of InVO4 for splitting of water
      Rakesh, K.; Khaire, S.; Bhange, D.; Dhanasekaran, P.; Deshpande, S. S.; Awate, S. V.; Gupta, N. M.
      Journal of Materials Science (2011), 46 (16), 5466-5476CODEN: JMTSAS; ISSN:0022-2461. (Springer)
      We report in this paper on microstructural, optical and photocatalytic properties of single-phase indium orthovanadates, as a function of doping at lattice sites. The UV-visible spectra of these samples exhibited intense UV-region bands at 250 and 350 nm, besides broad absorption band in visible region (350-700 nm). The wavelength at absorption edge and the intensity of visible absorption showed considerable increase on doping of an impurity, particularly at V or O lattice sites. Also, the samples gave rise to blue-green photoluminescence emission, with overriding bands at ca. 420, 450, 460 and 485 nm, on excitation at 240-420 nm wavelengths. The intensity of these fluorescence bands varied with excitation wavelength and impurity content of a sample. In deviation with several earlier studies, only oxygen and no hydrogen were produced during photocatalytic splitting of water, in the expts. conducted under visible light (>395 nm) and at a pH of ∼6.5. The O2 yield depended on the dispersed metal co-catalyst, impurity content and the addn. of methanol as sacrificial reagent. On the other hand, small quantities of hydrogen and no oxygen were evolved on UV-irradn. of pure water using metal/InVO4. These results are ascribed to flat band potentials and the doping-induced inter-band donor and acceptor charge trapping states of InVO4, the presence of which is revealed by XRD, luminescence and XPS studies. Our study also confirms that the onset of absorption edge may not necessarily correspond to band-to-band energy gap of a semiconducting material. This accounts for some anomalous band gap energies reported earlier for InVO4.
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      Wetchakun, N.; Wanwaen, P.; Phanichphant, S.; Wetchakun, K. Influence of Cu doping on the visible-light-induced photocatalytic activity of InVO4. RSC Adv. 2017, 7, 13911,  DOI: 10.1039/C6RA27138C
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      Influence of Cu doping on the visible-light-induced photocatalytic activity of InVO4
      Wetchakun, Natda; Wanwaen, Pimonrat; Phanichphant, Sukon; Wetchakun, Khatcharin
      RSC Advances (2017), 7 (23), 13911-13918CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The photocatalytic degrdn. of methylene blue (MB) in the presence of pure InVO4 or a 0.5-5.0 mol% Cu-doped InVO4 composite under visible light irradn. (λ ≥ 400 nm) was studied in this research. The structural and photophys. properties of the as-prepd. samples in the photocatalytic degrdn. process were investigated. The doping of InVO4 with a Cu photocatalyst results in wide absorption in the visible-light region and superior visible-light-driven photocatalytic activities in the degrdn. of MB. The results indicate that the InVO4 sample doped with 1.0 mol% of Cu shows the highest photocatalytic activity. The enhanced photocatalytic activity was attributed to the copper ions acting as trapping sites, facilitating the sepn. of charge carriers. The main active species for the degrdn. of MB were investigated to explain the enhancement of the photocatalytic performance of Cu-doped InVO4. A possible photocatalytic degrdn. pathway for aq. MB dye and a charge transfer mechanism for Cu-doped InVO4 were proposed.
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    22. 22
      Malingowski, A. C.; Stephens, P. W.; Huq, A.; Huang, Q.; Khalid, S.; Khalifah, P. G. Substitutional Mechanism of Ni into the Wide-Band-Gap Semiconductor InTaO4 and Its Implications for Water Splitting Activity in the Wolframite Structure Type. Inorg. Chem. 2012, 51, 6096– 6103,  DOI: 10.1021/ic202715c
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      22
      Substitutional Mechanism of Ni into the Wide-Band-Gap Semiconductor InTaO4 and Its Implications for Water Splitting Activity in the Wolframite Structure Type
      Malingowski, Andrew C.; Stephens, Peter W.; Huq, Ashfia; Huang, Qingzhen; Khalid, Syed; Khalifah, Peter G.
      Inorganic Chemistry (2012), 51 (11), 6096-6103CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The mechanism of Ni substitution into the oxide semiconductor InTaO4 has been studied through a combination of structural and spectroscopic techniques, providing insights into its previously reported photoactivity. Magnetic susceptibility and x-ray absorption near-edge spectroscopy (XANES) measurements demonstrate that nickel is divalent within the host lattice. The combined refinement of synchrotron x-ray and neutron powder diffraction data indicates that the product of Ni doping has the stoichiometry of (In1-xNi2x/3Tax/3)TaO4 with a soly. limit of x ≈ 0.18, corresponding to 12% Ni on the In site. Single-phase samples were only obtained at synthesis temps. of 1150° C or higher due to the sluggish reaction mechanism that is hypothesized to result from small free energy differences between (In1-xNi2x/3Tax/3)TaO4 compds. with different x values. Undoped InTaO4 is shown to have an indirect band gap of 3.96 eV, with direct optical transitions becoming allowed at photon energies in excess of 5.1 eV. Very small band-gap redns. (less than 0.2 eV) result from Ni doping, and the origin of the yellow color of (In1-xNi2x/3Tax/3)TaO4 compds. instead results from a weak 3A2g → 3T1g internal d → d transition not assocd. with the conduction or valence band that is common to oxide compds. with Ni2+ in an octahedral environment.
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    23. 23
      Su, L.; Fan, X.; Cai, G.; Jin, Z. Tunable luminescence properties and energy transfer of Tm3+, Dy3+, and Eu3+ co-activated InNbO4 phosphors for warm-white-lighting. Ceram. Int. 2016, 42, 15994– 16006,  DOI: 10.1016/j.ceramint.2016.07.105
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      23
      Tunable luminescence properties and energy transfer of Tm3+, Dy3+, and Eu3+ co-activated InNbO4 phosphors for warm-white-lighting
      Su, Liumei; Fan, Xing; Cai, Gemei; Jin, Zhanpeng
      Ceramics International (2016), 42 (14), 15994-16006CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
      A series of Dy3+/Tm3+ singly-doped, Dy3+/Tm3+ co-doped, and Dy3+/Tm3+/Eu3+ tri-doped InNbO4 phosphors have been synthesized by a high-temp. solid-state reaction method. Powder X-ray diffraction (XRD), structure refinement, UV-visible diffuse reflectance (UV-DR) spectrum, photoluminescence excitation (PLE) and emission (PL) spectrum, decay lifetimes and CIE chromaticity coordinate were employed to understand the origins of luminescence properties for synthesized phosphors. Rare-earth activators were detd. to be completely dissolved into the host lattice and occupied the 2e sites deviated from an inversion center by virtue of XRD, structure refinement and photoluminescence features. The photoluminescence excitation spectra of phosphors exhibited a broad excitation band ascribed to charge-transfer of O2--Nb5+, ranging from 200 to 300 nm, and some characteristic excitation peaks assocd. with energy level transition of RE ions. Under the excitation of UV light, phosphors presented characteristic emission originating from f-f transition within the 4f configuration of RE ions. With increasing Dy3+ concn., emission colors of InNbO4:Tm3+, Dy3+ phosphors can be appropriately tuned from blue to yellow, including almost all the white light region, by means of energy transfer from Tm3+ to Dy3+. Energy transfer rates and efficiencies were calcd. according to decay lifetimes and energy transfer mechanism followed a resonant type dipole-dipole interaction with crit. distance of 22.23 Å between Tm3+ and Dy3+. By compensating a red component of Eu3+ ion, single-component warm-white-light emitting was realized in InNbO4:Tm3+,Dy3+,Eu3+ phosphors. They might be promising candidates for color-tunable light-conversion components in the fabrication of UV-pumped warm-white-light emitting diodes (WLEDs) for solid-state lighting. Relationship of crystal structures and luminescence properties for InNbO4 and YNbO4 phosphors is as well explicated based on structural anal.
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      Tang, A.; Zhang, D. F.; Yang, L. Synthesis and luminescence properties of novel red emitting phosphor InNbO4:Eu3+ for white light emitting diodes. Russ. Chem. Bull. 2012, 61, 2172,  DOI: 10.1007/s11172-012-0304-2
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      Synthesis and luminescence properties of novel red-emitting phosphor InNbO4:Eu3+ for white-light emitting diodes
      Tang, A.; Zhang, D. F.; Yang, Liu
      Russian Chemical Bulletin (2012), 61 (11), 2172-2174CODEN: RCBUEY; ISSN:1066-5285. (Springer)
      Red-emitting phosphor InNbO4:Eu3+ was synthesized by the solid-state reaction. Its crystal structure, particle size distribution, and luminescence properties were studied. The powder X-ray diffraction pattern shows that pure InNbO4:Eu3+ was obtained. According to the spectra obtained, this phosphor can efficiently be excited with the light at wavelengths of 394 and 466 nm to emit the strong red radiation at 612 nm due to the 5D0→7F2 transition of Eu3+. The best results were obtained at the concn. of the Eu3+ dopant equal to 4 mol.%. The chromaticity parameters of InNbO4:0.04Eu3+ are close to stds. of the National Television Std. Committee. Thus, InNbO4:Eu3+ is a promising red-emitting phosphor for white-light emitting diodes.
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      Errandonea, D.; Ruiz-Fuertes, J. A brief review of the effects of pressure on wolframite-type oxides. Crystals 2018, 8, 71,  DOI: 10.3390/cryst8020071
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      A brief review of the effects of pressure on wolframite-type oxides
      Errandonea, Daniel; Ruiz-Fuertes, Javier
      Crystals (2018), 8 (2), 71/1-71/19CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
      In this article, we review the advances that have been made on the understanding of the high-pressure (HP) structural, vibrational, and electronic properties of wolframite-type oxides since the first works in the early 1990s. Mainly tungstates, which are the best known wolframites, but also tantalates and niobates, with an isomorphic ambient-pressure wolframite structure, have been included in this review. Apart from estg. the bulk moduli of all known wolframites, the cation-oxygen bond distances and their change with pressure have been correlated with their compressibility. The compn. variations of all wolframites have been employed to understand their different structural phase transitions to post-wolframite structures as a response to high pressure. The no. of Raman modes and the changes in the band-gap energy have also been analyzed in the basis of these compositional differences. The reviewed results are relevant for both fundamental science and for the development of wolframites as scintillating detectors. The possible next research avenues of wolframites under compression have also been evaluated.
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      Baran, E. J. Materials belonging to the CrVO4 structure type: preparation, crystal chemistry and physicochemical properties. J. Mater. Sci. 1998, 33, 2479,  DOI: 10.1023/A:1004380530309
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      Materials belonging to the CrVO4 structure type: preparation, crystal chemistry and physicochemical properties
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      Journal of Materials Science (1998), 33 (10), 2479-2497CODEN: JMTSAS; ISSN:0022-2461. (Chapman & Hall)
      A review, with ∼220 refs. Crystallog. data of vanadates, phosphates, chromates, sulfates and selenates belonging to the CrVO4 (β-CrPO4) structure type and of some of its most important polymorphs, are reported. Characteristic structural peculiarities and the effect of pressure on the phase transformations are discussed. A definitive stability field for this structure type could be established. Synthetic procedures for the prepn. of the different compds. adopting this and closely related structures are presented and its thermal, spectroscopic and magnetic properties are discussed. A brief overview on the most important characteristics of materials adopting the α-CrPO4 structure is also presented.
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      López-Moreno, S.; Rodríguez-Hernández, P.; Muñoz, A.; Errandonea, D. First-Principles Study of InVO4 under Pressure: Phase Transitions from CrVO4 to AgMnO4-Type Structure. Inorg. Chem. 2017, 56, 2697– 2711,  DOI: 10.1021/acs.inorgchem.6b02867
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      First-Principles Study of InVO4 under Pressure: Phase Transitions from CrVO4- to AgMnO4-Type Structure
      Lopez-Moreno, Sinhue; Rodriguez-Hernandez, Placida; Munoz, Alfonso; Errandonea, Daniel
      Inorganic Chemistry (2017), 56 (5), 2697-2711CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      First-principles calcns. were carried out to study the InVO4 compd. under pressure. Total energy calcns. were performed to analyze the structural behavior of the exptl. known polymorphs of InVO4: α-MnMoO4-type (I), CrVO4-type (III), and wolframite (V). The results about the stability of this compd. beyond the pressures reached by expts. are presented. Some new high-pressure phases are proposed from the study of 13 possible candidates. The quasi-harmonic approxn. was used to calc. the sequence of phase transitions at 300 K: CrVO4-type, III (the transition pressure is given in parentheses) → wolframite, V (4.4 GPa) → raspite, VI (28.1 GPa) → AgMnO4-type, VII (44 GPa). Equations of state and phonon frequencies as a function of pressure were calcd. for the studied phases. To det. the stability of each phase, the authors also report the phonon dispersion along the Brillouin zone and the phonon d. of states for the most stable polymorphs. The electronic band structure for the low- and high-pressure phases for the studied polymorphs is presented as well as the pressure evolution of the band gap by using the HSE06 hybrid functional.
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      Blasse, G.; Bril, A. Luminescence of Phosphors Based on Host Lattices ABO4 (A is Sc, In; B is P, V, Nb). J. Chem. Phys. 1969, 50, 2974,  DOI: 10.1063/1.1671493
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      Luminescence of phosphors based on host lattices ABO4 (A is Sc, In; B is P, V, Nb)
      Blasse, G.; Bril, Alfred
      Journal of Chemical Physics (1969), 50 (7), 2974-80CODEN: JCPSA6; ISSN:0021-9606.
      The luminescence of the unactivated and Bi3+- and Eu3+-activated compds. ABO4 (A is Sc, In; B is P, V, Nb) is reported and compared with the luminescence of the corresponding rare-earth and Y compds. The ScNbO4 shows a weak, blue emission, ScVO4 a blue-green, and InVO4 a green emission. The other host lattices show no or only a very weak emission at room temp. The color of the vanadate emission depends strongly on the choice of A. This was studied by replacing part of the A ions by other ions or a combination of ions. The shift of the vanadate emission is ascribed to 2 effects, viz., metal-metal interaction (e.g., Bi3+-VO4) and the effect of the size of the metal ions at the A sites (e.g., Sc3+). The Eu3+-activated phosphors show some interesting features. The intensity of the forced elec. dipole emission from the Eu3+ ion seems to depend strongly on the position of the lowest strong absorption band. In the zircon structure the 5D0-7F2 emission is hypersensitive to the surroundings, although no linear crystal-field component can be present. Both (In, Eu)PO4 and (In, Eu)VO4 show an orange (5D0-7F1) emission due to the presence of inversion symmetry at the In3+ site.
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      Tang, A.; Gu, L.; Shao, F.; Liu, X.; Zhao, Y.; Chen, H.; Zhang, H. Influence of Bi3+ content on photoluminescence of InNbO4:Eu3+, Bi3+ for white light-emitting diodes. Mater. Sci.-Pol. 2017, 35, 435– 439,  DOI: 10.1515/msp-2017-0053
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      Influence of Bi3+ content on photoluminescence of InNbO4:Eu3+,Bi3+ for white light-emitting diodes
      Tang, An; Gu, Liduo; Shao, Fengxiang; Liu, Xidong; Zhao, Yongtao; Chen, Haijun; Zhang, Hongsong
      Materials Science-Poland (2017), 35 (2), 435-439CODEN: MSACDV; ISSN:2083-134X. (Walter de Gruyter GmbH)
      A series of red-emitting phosphors InNbO4:Eu3+,Bi3+ was prepd. by a high temp. solid-state reaction. The structure, size distribution and luminescence properties of the phosphors were resp. characterized by X-ray diffraction (XRD), laser particle size and mol. fluorescence spectrometer. The XRD results indicate that the phase-pure samples have been obtained and the crystal structure of the host has not changed under the Eu3+ and Bi3+ co-doping. The test of size distribution shows that the phosphor has a normal size distribution. The excitation spectra illustrate that the dominant sharp peaks are located at 394 nm (7F0→5L6) and 466 nm (7F0→5D2). Meanwhile, the emission spectra reveal that the phosphors excited by the wavelength of 394 nm or 466 nm have an intense red-emission line at 612 nm owing to the 5D0→7F2 transition of Eu3+. Bi3+ doping has not changed the peak positions except the photoluminescence intensity. The emission intensity is related to Bi3+ concn., and it is up to the max. when the Bi3+-doping concn. is 4 mol%. Due to good photoluminescence properties of the phosphor, the InNbO4:0.04Eu3+,0.04Bi3+ may be used as a red component for white light-emitting diodes.
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      Shia, Z.-R.; Chen, H.-L.; Tsai, Y.-Y.; Wu, S.; Chang, Y.-S. Synthesis and Photoluminescence Properties of InVO4:Eu3+ Phosphors Prepared using Sol–Gel Method. ECS Trans. 2010, 28, 145– 154,  DOI: 10.1149/1.3367220
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      Brixner, L. H.; Chen, H.-Y. On the structural and luminescent properties of the InTa1–xNbxO4 system. Mater. Res. Bull. 1980, 15, 607– 612,  DOI: 10.1016/0025-5408(80)90140-3
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      On the structural and luminescent properties of the indium tantalum niobate (InTa1-xNbxO4) system
      Brixner, L. H.; Chen, H. Y.
      Materials Research Bulletin (1980), 15 (5), 607-12CODEN: MRBUAC; ISSN:0025-5408.
      InNbO4 and InTaO4 crystallize in the monoclinic space group P2/a of the wolframite structure and unlimited solid solns. are formed over the entire range of x. The cell vol. decreases with increasing Nb content. Both the niobate and tantalate are UV-excitable blue-emitting phosphors with the emission centered around 403 nm in a broad band of ∼85 nm half width. In the pure state, InTaO4 is a more efficient phosphor than InNbO4, max. quantum yield being obsd. at x = 0.075. Eu3+- and Tb3+-doped InTaO4 also exhibit luminescent properties.
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      Shen, J.; Yang, H.; Shen, Q.; You, Z. Synthesis and Characterization of InVO4 Nano-materials and their Photoluminescence Properties. Procedia Eng. 2014, 94, 64– 70,  DOI: 10.1016/j.proeng.2013.11.043
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      Synthesis and Characterization of InVO4 Nano-materials and their Photoluminescence Properties
      Shen, Jianchao; Yang, Hui; Shen, Qianhong; You, Zengyu
      Procedia Engineering (2014), 94 (), 64-70CODEN: PERNBE; ISSN:1877-7058. (Elsevier Ltd.)
      InVO4 nano-materials with various morphologies were prepd. via hydrothermal method. X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscope (SEM) were adopted to investigate the effects of various factors, such as pH value, V/In molar ratio, hydrothermal conditions and different surfactants, on the phase and morphol. of InVO4. Photoluminescence spectra were also employed to est. the photoluminescence properties of as-prepd. InVO4 samples. The results show that the pH value and V/In molar ratio have an important influence on the phase of InVO4, while the hydrothermal conditions and surfactants mainly affect the morphol. of InVO4. As-prepd. InVO4 samples display an intensive visible emission located in the green region with its max. intensity peak at 530 nm, and the morphol. and grain size have obvious effect on the fluorescence intensity of InVO4.
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      Zeng, G.-S.; Yu, J.; Zhu, H.-Y.; Liu, H.-L.; Xing, Q.-J.; Bao, S.-K.; He, S.; Zou, J.-P.; Au, C.-T. Controllable synthesis of InTaO4 catalysts of different morphologies using a versatile sol precursor for photocatalytic evolution of H2. RSC Adv. 2015, 5, 37603,  DOI: 10.1039/C5RA03638K
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      Controllable synthesis of InTaO4 catalysts of different morphologies using a versatile sol precursor for photocatalytic evolution of H2
      Zeng, Gui-Sheng; Yu, Jian; Zhu, Hong-Yun; Liu, Hui-Long; Xing, Qiu-Ju; Bao, Shao-Kui; He, Shun; Zou, Jian-Ping; Au, Chak-Tong
      RSC Advances (2015), 5 (47), 37603-37609CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      For the first time, InTaO4 photocatalysts with different morphologies and structures were facilely prepd. using a versatile sol precursor. SEM, TEM and HRTEM were used to characterize their structures and morphologies. The as-synthesized materials exhibit different physicochem. properties. Compared to InTaO4 nanoparticles and SiO2@InTaO4 core-shell nanospheres, InTaO4 nanofibers have higher light absorption ability, larger sp. surface area, and better photocatalytic activity for hydrogen evolution. We proposed mechanisms for the formation and photocatalytic activity of the three catalysts. The present work not only provides a new approach for the synthesis of tantalates that are different in morphol. and structure but also offers new insights into the controllable prepn. of photocatalysts through a versatile precursor for environmental and energy applications.
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      Lacomba-Perales, R.; Ruiz-Fuertes, J.; Errandonea, D.; Martínez-García, D.; Segura, A. Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius. EPL (Eur. Lett.) 2008, 83, 37002,  DOI: 10.1209/0295-5075/83/37002
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      Carnall, W. T.; Goodman, G. L.; Rajnak, K.; Rana, R. S. A systematic analysis of the spectra of the lanthanides doped into single crystal LaF3. J. Chem. Phys. 1989, 90, 3443,  DOI: 10.1063/1.455853
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      A systematic analysis of the spectra of the lanthanides doped into single crystal lanthanum fluoride (LaF3)
      Carnall, W. T.; Goodman, G. L.; Rajnak, K.; Rana, R. S.
      Journal of Chemical Physics (1989), 90 (7), 3443-57CODEN: JCPSA6; ISSN:0021-9606.
      The optical (4000-15,000 cm-1) spectra of the lanthanides doped into single crystal LaF3 were interpreted in terms of transitions within 4fN configurations. Energy matrixes combining free-ion terms with a crystal field for an approx. model which assumes C2v instead of the actual C2 site symmetry were diagonalized. Correlations were obtained between exptl. transition energies and the computed level structures. The results are reported of previously unpublished exptl. spectroscopic investigations of Nd3+ and Sm3+:LaF3, and predicted energy levels for Pm3+bLaF3. The spectroscopic data for each ion were independently interpreted using an effective-operator model, then the model parameters were intercompared. Systematic trends were identified, and a comprehensive energy level diagram is presented.
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      Gruber, J. B.; Nash, K. L.; Yow, R. M.; Sardar, D. K.; Valiev, U. V.; Uzokov, A. A.; Burdick, G. W. Spectroscopic and magnetic susceptibility analyses of the 7FJ and 5D4 energy levels of Tb3+(4f8) in TbAlO3. J. Lumin. 2008, 128, 1271– 1284,  DOI: 10.1016/j.jlumin.2007.12.041
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      Spectroscopic and magnetic susceptibility analyses of the 7FJ and 5D4 energy levels of Tb3+(4f8) in TbAlO3
      Gruber, John B.; Nash, Kelly L.; Yow, Raylon M.; Sardar, Dhiraj K.; Valiev, Uygun V.; Uzokov, Abdulla A.; Burdick, Gary W.
      Journal of Luminescence (2008), 128 (8), 1271-1284CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
      Detailed analyses of spectroscopic and temp.-dependent magnetic susceptibility data are reported for the crystal-field split energy levels of the 7FJ and 5D4 of Tb3+ in stoichiometric single crystals of ortho-aluminate TbAlO3. The spectroscopic data include absorption spectra obtained between 2940-480 nm from 8-300 K. High resoln. fluorescence spectra are reported, representing transitions from 5D4 to 7F6,5,4, at a sample temp. of 85 K. Using crystal-field modeling techniques recently adapted for low symmetry systems, we have assigned all 58 exptl. Stark levels within the 7FJ and 5D4 manifolds, with a fitting std. deviation of 4.5 cm-1 (3.8 cm-1 rms error). As a further test, the theor. Stark levels and calcd. wavefunctions were used to det. the temp. dependence of the magnetic susceptibility along the c-axis of the TbAlO3 crystal. Good agreement is obtained between the calcd. susceptibility and temp.-dependent magnetic data reported earlier, including a prediction of a 0.2 cm-1 splitting of the ground-state quasi-doublet. The susceptibility calcn. also confirms the predicted ordering of states within the 7F6 multiplet manifold.
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      Cascales, C.; Zaldo, C. Spectroscopic Characterization and Systematic Crystal-Field Modeling of Optically Active Rare Earth RE3+ Ions in the Bismuth Germanate BiY1-xRxGeO5 Host. Chem. Mater. 2006, 18, 3742– 3753,  DOI: 10.1021/cm060785t
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      Spectroscopic Characterization and Systematic Crystal-Field Modeling of Optically Active Rare Earth R3+ Ions in the Bismuth Germanate BiY1-xRxGeO5 Host
      Cascales, C.; Zaldo, C.
      Chemistry of Materials (2006), 18 (16), 3742-3753CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The synthesis conditions and structural characteristics of polycryst. R-doped BiY1-xRxGeO5 or stoichiometric BiRGeO5 (R = trivalent rare earth Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, or Yb3+) germanate compds. are presented. The energy levels of each R3+ up to the absorption edge of the germanate matrix have been established from 10 K optical absorption and photoluminescence spectra measurements. Detailed single-electron C2(Cs) Hamiltonians combining free-ion and crystal-field interactions have been used to model the above 4fN configurations. Very satisfactory correlations were obtained between obsd. and the calcd. energy levels, and corresponding tabulated data are reported. Systematic trends in the evolution of crystal-field parameters have been identified. The proper estn. of phenomenol. crystal-field parameters and crystal-field strengths at the R3+ site for the whole Pr-Yb series is the first step in prospecting the possibilities of this germanate structure as a new solid-state laser host. The obsd. medium-high (∼660 cm-1) splitting for the 2F7/2 ground state of Yb3+ in BiYbGeO5 fulfills a prerequisite for an efficient Yb quasi-three-level laser operation. The possibility of accepting large amts. of optically active R3+ ions and the intense Raman shifts obsd. at high frequencies are other advantageous characteristics of the title host.
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      Porcher, P.; Couto Dos Santos, M.; Malta, O. Relationship between phenomenological crystal field parameters and the crystal structure: The simple overlap model. Phys. Chem. Chem. Phys. 1999, 1, 397– 405,  DOI: 10.1039/a803807d
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      Relationship between phenomenological crystal field parameters and the crystal structure: The simple overlap model
      Porcher, Pierre; Couto Dos Santos, Marcos; Malta, Oscar
      Physical Chemistry Chemical Physics (1999), 1 (3), 397-405CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The simple overlap model was used to calc. crystal field parameters from the crystal structure. It is applied to 25 rare earth and some actinide and 3d transition element compds. for which crystal field parameters were previously detd. exptl. The model is based on effective charges, located around the middle of the rare earth ion-ligand distance, which are assumed to be proportional to the magnitude of the overlap, ρ, between the rare earth ion and ligand wavefunctions. Good agreement between theor. and exptl. crystal field parameters is found for most of the cases studied. The best predictions were obtained when the overlap varies slightly from one rare earth compd. to another (0.05<ρ<0.08), in agreement with the general assumption that the f orbitals have a small part in the chem. bonding. For the two examples involving the 5f elements, the overlaps with the ligand have, curiously, the same magnitude as in the case of the rare earth elements. As expected, the 3d element wavefunctions show a stronger mixing with those of the ligands (0.10<ρ<0.25).
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      Hernández-Rodríguez, M. A.; Muñoz-Santiuste, J. E.; Lavín, V.; Lozano-Gorrín, A. D.; Rodríguez-Hernández, P.; Muñoz, A.; Venkatramu, V.; Martín, I. R.; Rodríguez-Mendoza, U. R. High pressure luminescence of Nd3+ in YAlO3 perovskite nanocrystals. A crystal-field analysis. J. Chem. Phys. 2018, 148, 044201  DOI: 10.1063/1.5010150
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      High pressure luminescence of Nd3+ in YAlO3 perovskite nanocrystals: A crystal-field analysis
      Hernandez-Rodriguez, Miguel A.; Munoz-Santiuste, Juan E.; Lavin, Victor; Lozano-Gorrin, Antonio D.; Rodriguez-Hernandez, Placida; Munoz, Alfonso; Venkatramu, Vemula; Martin, Inocencio R.; Rodriguez-Mendoza, Ulises R.
      Journal of Chemical Physics (2018), 148 (4), 044201/1-044201/10CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      Pressure-induced energy blue- and red-shifts of the 4F3/2 → 4I9/2,11/2 near-IR emission lines of Nd3+ ions in YAlO3 perovskite nano-particles have been measured from ambient conditions up to 29 GPa. Different pos. and neg. linear pressure coeffs. have been calibrated for the emission lines and related to pressure-induced changes in the interactions between those Nd3+ ions and their twelve oxygen ligands at the yttrium site. Potentiality of the simple overlap model, combined with ab initio structural calcns., in the description of the effects of these interactions on the energy levels and luminescence properties of the optically active Nd3+ ion is emphasized. Simulations show how the energies of the 4f3 ground configuration and the barycenters of the multiplets increase with pressure, whereas the Coulomb interaction between f-electrons decreases and the crystal-field strength increases. All these effects combined explain the wavelength blue-shifts of some near-IR emission lines of Nd3+ ions. Large pressure rates of various emission lines suggest that a YAlO3 perovskite nano-crystal can be a potential candidate for near-IR optical pressure sensors. (c) 2018 American Institute of Physics.
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      Chang, N. C.; Gruber, J. B.; Leavitt, R. P.; Morrison, C. A. Optical spectra, energy levels, and crystal-field analysis of tripositive rare-earth ions in Y2O3. II. Kramers ions in C2 sites. J. Chem. Phys. 1982, 76, 3877,  DOI: 10.1063/1.443530
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      Optical spectra, energy levels, and crystal field analysis of tripositive rare earth ions in yttrium oxide. I. Kramers ions in C2 sites
      Chang, N. C.; Gruber, John B.; Leavitt, Richard P.; Morrison, Clyde A.
      Journal of Chemical Physics (1982), 76 (8), 3877-89CODEN: JCPSA6; ISSN:0021-9606.
      Optical absorption and fluorescence spectra of the Kramers ions Ce3+, Sm3+, Dy3+, and Yb3+ in the C2 sites of Y2O3 are reported. A crystal-field anal. of these data and previously reported data for Nd3+ and Er3+ was performed that is based on a Hamiltonian of C2 point-group symmetry, including J-mixing effects. A best-fit anal. was performed for the ions Nd3+, Sm3+, Dy3+, and Er3+; resulting in root-mean square deviations between calcd. and exptl. levels range from 5.1 to 7.9 cm-1. Results of this anal. were used to obtain a smoothed set of crystal-field parameters for the entire lanthanide series; these smoothed parameters were used to predict the energy levels of Ce3+, Yb3+, and Gd3+ in Y2O3. Predictions for Ce3+ and Yb3+ were compared with exptl. data.
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      Gluba, M. A.; Nickel, N. H.; Karpensky, N. Interstitial zinc clusters in zinc oxide. Phys. Rev. B 2013, 88, 245201,  DOI: 10.1103/PhysRevB.88.245201
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      Physical Review B: Condensed Matter and Materials Physics (2013), 88 (24), 245201/1-245201/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Doped zinc oxide (ZnO) exhibits anomalous Raman modes in the range of 270 to 870 cm-1. Commonly, the resonance at 275 cm-1 is attributed to the local vibration of Zn atoms in the vicinity of extrinsic dopants. We revisit this assignment by investigating the influence of isotopically purified zinc oxide thin films on the frequency of the vibrational mode around 275 cm-1. For this purpose, undoped and nitrogen-doped ZnO thin-films with Zn isotope compns. of natural Zn, 64Zn, 68Zn, and a 1:1 mixt. of 64Zn and 68Zn were grown by pulsed laser deposition. The isotopic shift and the line shape of the Raman resonance around 275 cm-1 are analyzed in terms of three different microscopic models, which involve the vibration of (i) interstitial zinc atoms bound to extrinsic defects, (ii) interstitial diat. Zn mols., and (iii) interstitial zinc clusters. The energy diagram of interstitial Zn-Zn bonds in a ZnO matrix is derived from d. functional theory calcns. The interstitial Zn-Zn bond is stabilized by transferring electrons from the antibonding orbital into the ZnO conduction band. This mechanism facilitates the formation of interstitial Zn clusters and fosters the common n-type doping asymmetry of ZnO.
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      Mondal, S.; Appalakondaiah, S.; Vaitheeswaran, G. High pressure structural, electronic, and optical properties of polymorphic InVO4 phases. J. Appl. Phys. 2016, 119, 085702  DOI: 10.1063/1.4942182
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      High pressure structural, electronic, and optical properties of polymorphic InVO4 phases
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      Journal of Applied Physics (Melville, NY, United States) (2016), 119 (8), 085702/1-085702/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      In the present work, we report a detailed d. functional theory calcn. on polymorphic InVO4 phases by means of projector augmented wave method. The computed first-order structural phase transformation from orthorhombic (Cmcm) to monoclinic (P2/c) structure is found to occur around 5.6 GPa along with a large vol. collapse of 16.6%, which is consistent with previously reported exptl. data. This transformation also leads to an increase in the coordination no. of vanadium atom from 4 to 6. The computed equil. and high pressure structural properties of both InVO4 phases, including unit cell parameters, equation of state, and bulk moduli, are in good agreement with the available exptl. data. In addn., compressibility is found to be highly anisotropic and the b-axis being more compressible than the other for both the structures. Electronic band structures for both the phases were calcd., and the band gaps for orthorhombic and monoclinic InVO4 are found to be 4.02 and 1.67 eV, resp., within the Tran-Blaha Modified Becke-Johnson potential as implemented in linearized augmented planewave method. We further examd. the optical properties such as dielec. function, refractive index, and absorption spectra for both the structures. From the implications of these results, it can be proposed that the high pressure InVO4 phase can be more useful than orthorhombic phase for photo catalytic applications. (c) 2016 American Institute of Physics.
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      Zhou, J.; Huang, F.; Xu, J.; Chen, H.; Wang, Y. Luminescence study of a self-activated and rare earth activated Sr3La(VO4). J. Mater. Chem. C 2015, 3, 3023– 3028,  DOI: 10.1039/C4TC02783C
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      Luminescence study of a self-activated and rare earth activated Sr3La(VO4)3 phosphor potentially applicable in W-LEDs
      Zhou, Jiangcong; Huang, Feng; Xu, Ju; Chen, Hui; Wang, Yuansheng
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2015), 3 (13), 3023-3028CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      A novel Sr3La(VO4)3 phosphor is explored which exhibits the bright self-activated luminescence and the ability to sensitize emission of rare earth activators, under the excitation of near UV light. The self-activated emission of the Sr3La(VO4)3 host is peaked at 520 nm with a broad emission band ranging from 400 nm to 700 nm, ascribing to the charge transfer in (VO4)3- groups; while the Sr3La(VO4)3:Eu3+ phosphors show both the broad band luminescence of (VO4)3- groups and the sharp peak emissions of Eu3+ ions. Through doping concn. optimization, intense warm white light is achieved under near UV excitation, with a quantum yield of 31.2%. This phosphor is therefore potentially applicable in near UV chip excited white LEDs.
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      Ronde, H.; Blasse, G. The nature of the electronic transitions of the vanadate group. J. Inorg. Nucl. Chem. 1978, 40, 215– 219,  DOI: 10.1016/0022-1902(78)80113-4
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      The luminescence of inorg. vanadates, e.g. NaCaVO4, Zn3(VO4)2, and LiMgP1-xVxO4 (x = 0.01, 0.05), was examd. and the obsd. emission assigned to the 3T1(t152e) → 1A1(t16) and 3T2(t152e) → 1A1 transitions. The 3T1(t152e) level is the lowest excited state of the VO4 group. The excitation was assigned to the 1A1(t16) → 1T1(t152e) and → 1T2(t152e) transitions. The position of the excitation bands is only slightly influenced by the host. The host-lattice dependence of the quenching temp. of the emission is discussed in terms of a configurational-coordinate model.
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      Nakajima, T.; Isobe, M.; Tsuchiya, T.; Ueda, Y.; Manabe, T. Correlation between Luminescence Quantum Efficiency and Structural Properties of Vanadate Phosphors with Chained, Dimerized, and Isolated VO4 Tetrahedra. J. Phys. Chem. C 2010, 114, 5160– 5167,  DOI: 10.1021/jp910884c
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      Correlation between Luminescence Quantum Efficiency and Structural Properties of Vanadate Phosphors with Chained, Dimerized, and Isolated VO4 Tetrahedra
      Nakajima, Tomohiko; Isobe, Masahiko; Tsuchiya, Tetsuo; Ueda, Yutaka; Manabe, Takaaki
      Journal of Physical Chemistry C (2010), 114 (11), 5160-5167CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The internal luminescence quantum efficiency and color properties of AVO3 (A = Li, Na, K, Rb, and Cs), M2V2O7 (M = Mg, Ca, Sr, Ba, and Zn), and M3V2O8 (M = Mg, Ca, Sr, Ba, and Zn) were studied. These vanadate phosphors exhibited broadband emission from 400 nm to over 800 nm due to the 1-electron charge transfer transition in the VO4 tetrahedra, and the color of the luminescent materials ranged from green to yellow-orange via white, corresponding to 0.277 < x < 0.494 and 0.389 < y < 0.488 on the CIE chromaticity diagram. The luminescence quantum efficiency of the vanadate phosphors with VO4 tetrahedra was strongly enhanced by the strong interaction between V ions and the weak interaction between V and A(M) ions in the crystal structures. The authors hypothesize that the long exciton diffusion lifetime induced by these structural features enhanced luminescence, leading to high quantum efficiency.
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      Van de Krol, R.; Ségalini, J.; Enache, C. S. Influence of point defects on the performance of InVO4 photoanodes. J. Photonics Energy 2011, 1, 016001  DOI: 10.1117/1.3564926
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      Influence of point defects on the performance of InVO4 photoanodes
      van de Krol, Roel; Segalini, Julie; Enache, Cristina S.
      Journal of Photonics for Energy (2011), 1 (), 016001/1-016001/10CODEN: JPEOBV; ISSN:1947-7988. (Society of Photo-Optical Instrumentation Engineers)
      The properties of thin film InVO4 photoanodes for water splitting have been studied. Compact films of InVO4 were prepd. by spray pyrolysis and are found to be stable between pH 3 and 11. Although the indirect bandgap is 3.2 eV, a modest amt. of visible light absorption is obsd. The origin of this absorption is attributed to the presence of deep donor states at ∼0.7 eV below the conduction band. These donor states presumably correspond to oxygen vacancies, which form as a result of small but unavoidable deviations of In:V from the ideal 1:1 stoichiometry during the wet-chem. synthesis process. Shallow donors are absent in this material, in contrast to what is normally obsd. for metal oxides. The deep donor model explains the much stronger visible light absorption of powders compared to thin films. The defect chem. reactions that lead to the formation of the deep donors are shown, and are supported by photoluminescence data.
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      Nakajima, T.; Isobe, M.; Tsuchiya, T.; Ueda, Y.; Manabe, T. Photoluminescence property of vanadates M2V2O7 (M: Ba, Sr and Ca). Opt. Mater. 2010, 32, 1618– 1621,  DOI: 10.1016/j.optmat.2010.05.021
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      Photoluminescence property of vanadates M2V2O7 (M: Ba, Sr and Ca)
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      We investigated the photoluminescent properties including the internal quantum efficiency (η) for the M2V2O7 (M: Ba, Sr, and Ca) as a new vanadate phosphor system. In the M2V2O7, the broad band emission spectra from 400 to 800 nm derived from the charge transfer transition in the VO4 tetrahedra were obsd. The emission band of these compds. consisted of two maxima Em1 and Em2 corresponding to 3 T 1-1 A 1 and 3 T 2-1 A 1 transitions, resp. These two maxima red-shifted with the decreasing ionic radius of M-site, as a result, the luminescent color was varied from green (M: Ba) to yellowish orange (M: Ca). The η for the Ba2V2O7, Sr2V2O7 and Ca2V2O7 were 25%, 8% and 0.9%, resp. This enhancement of η in the M2V2O7 could be due to following structural features: the stronger V-V and weaker V-M correlations.
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      Delosh, R. G.; Tien, T. Y.; Gibbons, E. F.; Zacmanidis, P. J.; Stadler, H. L. Strong Quenching of Tb3+ Emission by Tb–V Interaction in YPO4–YVO4. J. Chem. Phys. 1970, 53, 681,  DOI: 10.1063/1.1674044
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      Strong quenching of Tb3+ emission by terbium-vanadium interaction in yttrium phosphate--yttrium vanadate
      DeLosh, Robert G.; Tien, T. Y.; Gibbons, E. F.; Zacmanidis, P. J.; Stadler, H. L.
      Journal of Chemical Physics (1970), 53 (2), 681-5CODEN: JCPSA6; ISSN:0021-9606.
      Solid solns. in the system YPO4-YVO4, with and without 2 mole % of Tb3+, were studied to det. the reason for the total inactivity of Tb in YVO4. The comparison of the diffuse reflectivity of Tb-contg. solns. with that of Tb-free ones shows a uv absorption at energies below the absorption edge of YVO4 in materials contg. both Tb and V. Selective excitation directly into Tb3+ in these wavelength regions produced reduced Tb3+ emission when there was V present. The band emission of the VO43+ groups, and the Tb3+ emission under electron beam excitation, were strongly quenched whenever Tb and V were both present. Emission of Dy3+, present as a 10-ppm impurity in all samples, was also quenched by the presence of both Tb and V, but not by either alone. All these observations imply energy loss by Tb-V interaction. A particular loss mechanism-the well-known intervalence transfer of charge from Tb to V resulting in an abrupt change of bond strength leading to a transfer of electronic excitation into lattice vibrations-is supported by measurements of Tb activity as a function of V concn. The intensity of Tb emission is proportionatel to the probability that none of 4 metal ion sites about a Tb is occupied by a V. This behavior suggests the intervalence transfer absorption mechanism, because only 4 of the nearest 6 metal ion sites have Tb-O-M bond angles (about 150°) consistent with good charge transfer from Tb to V; the remaining 2 sites have a Tb-O-M angle near 90° and should be relatively inactive in charge transfer.
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      Philips Research Reports (1967), 22 (5), 481-504CODEN: PRREA9; ISSN:0031-7918.
      The fluorescence of many oxide, oxyhalide, and fluoride phosphors; activated by Tb(III), was studied. The dependence of the spectral energy distribution of the emission on the surroundings is much weaker than for Eu(III) activated phosphors. This may be caused by the absence of charge-transfer absorption bands in the uv. region for Tb(III)-activated oxides. The position of the 4f-5d absorption band depends strongly on the host lattice. Factors influencing the position of this band are discussed, esp. crystal-field splitting in the 5d levels. Energy transfer from a host lattice to the Tb(III) ion is less efficient than that to Eu(III). Several explanations of this phenomenon are given. The max. quantum efficiency in the uv region is detd. by deviation from inverse symmetry at the Tb(III) site and the position of the 4f-5d absorption band of the Tb(III) ion. The larger the deviation and the longer the wavelength of the 4f-5d band, the lower is the efficiency.
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      Huignard, A.; Gacoin, T.; Boilot, J.-P. Synthesis and Luminescence Properties of Colloidal YVO4:Eu Phosphors. Chem. Mater. 2000, 12, 1090– 1094,  DOI: 10.1021/cm990722t
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      Synthesis and Luminescence Properties of Colloidal YVO4:Eu Phosphors
      Huignard, Arnaud; Gacoin, Thierry; Boilot, Jean-Pierre
      Chemistry of Materials (2000), 12 (4), 1090-1094CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Concd. colloidal solns. of well-dispersed YVO4:Eu nanoparticles are synthesized by pptn. reactions at room temp. and stabilized by Na hexametaphosphate. x-ray diffraction and electron microscopy characterizations show that the cryst. nanoparticles exhibit an ellipsoidal form with 2 characteristic dimensions of ∼15 and 30 nm. In comparison with the bulk, a lower luminescence efficiency as well as a higher concn. quenching are obsd. These deviations are explained as the variations of some characteristics of the colloidal samples, such as the crystallinity and the surface chem. When these parameters are optimized, the quantum yield of the luminescence reaches 38% for the nanoparticles contg. a Eu concn. of 15%.
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      Energy transfer mechanisms in Yb3+ doped YVO4 near-infrared downconversion phosphor
      Wei, Xian-Tao; Huang, Shan; Chen, Yong-Hu; Guo, Chang-Xin; Yin, Min; Xu, Wu
      Journal of Applied Physics (2010), 107 (10), 103107/1-103107/5CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Upon UV light excitation, an intense near-IR (NIR) emission of Yb3+(2F5/2 → 2F7/2) around 980 nm is obsd. in YVO4:Yb3+ phosphors. Owing to host absorption of YVO4, a broad excitation band ranging from 250 to 350 nm is recorded when the Yb3+ emission was monitored, which suggests an efficient energy transfer from host to Yb3+ ions. The Yb3+ concn. dependence of the visible vanadate emission as well as the Yb3+ emission is investigated. The decay curve of vanadate emission is measured under the excitation of a 266 nm pulsed laser. The decay time of the vanadate emission at 500 nm is remarkably reduced by introducing Yb3+ ions, further verifying that the energy transfer from the vanadate host to the Yb3+ ions is very efficient. Cooperative energy transfer (CET) is discussed as a possible mechanism for the NIR emission. The YVO4:Yb3+ phosphor can convert each UV photon into two NIR photons via CET, which has potential application in the high efficiency silicon-based solar cells. (c) 2010 American Institute of Physics.
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      Li, G. L.; Yin, Z. Theoretical insight into the electronic, optical and photocatalytic properties of InMO4 (M = V, Nb, Ta) photocatalysts. Phys. Chem. Chem. Phys. 2011, 13, 2824– 2833,  DOI: 10.1039/B921143H
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      Theoretical insight into the electronic, optical and photocatalytic properties of InMO4 (M = V, Nb, Ta) photocatalysts
      Li, Guo-Ling; Yin, Zhen
      Physical Chemistry Chemical Physics (2011), 13 (7), 2824-2833CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The electronic and optical properties of InMO4 (M = V, Nb, Ta) photocatalysts are studied using first-principles calcns. For all InMO4, the calcd. band gaps are larger than the measured optical gaps, indicating the existence of sub-bandgap transitions. Impurity states and excitons are considered to interpret the characteristic absorption onsets in the measured UV-visible diffuse reflection spectra. The novel visible-light-active water-splitting photocatalytic properties of InMO4 are related to the sub-bandgap transitions. Correlation between the impurity states and the photocatalytic activities is discussed for InMO4via the conventional mechanism of photocatalytic water-splitting on semiconductors. An excitonic mechanism analogous to Photosystem II in plant photosynthesis is also proposed for the photocatalytic water-splitting process on InMO4.
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      Feng, H.; Hou, D.; Huang, Y.; Hu, X. Facile synthesis of porous InNbO4 nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties. J. Alloys Compd. 2014, 592, 301– 305,  DOI: 10.1016/j.jallcom.2013.12.261
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      Facile synthesis of porous InNbO4 nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties
      Feng, Huiyu; Hou, Dongfang; Huang, Yunhui; Hu, Xianluo
      Journal of Alloys and Compounds (2014), 592 (), 301-305CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      Porous InNbO4 nanofibers with diams. of 50-100 nm were prepd. by a facile electrospinning method combined with subsequent annealing. The resulting photocatalyst was characterized by x-ray diffraction (XRD), SEM, transmission electron microscopy (TEM), UV-visible diffuse reflectance spectra (UV-vis DRS), and nitrogen-sorption anal. The photocatalytic activity of the photocatalysts was evaluated by degrdn. of Rhodamine B under visible-light irradn. Results demonstrate that the photocatalytic activity of the as-formed porous InNbO4 nanofibers by electrospinning is improved, in comparison to that of the InNbO4 crystallites that were prepd. by a high-temp. solid-state reaction. The enhanced visible-light-driven photocatalytic activity is attributed to the porous nanofibrous architecture, high surface area, and narrower band gap.
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      Back, M.; Massari, A.; Boffelli, M.; Gonella, F.; Riello, P.; Cristofori, D.; Riccò, R.; Enrichi, F. Optical investigation of Tb3+-doped Y2O3 nanocrystals prepared by Pechini-type sol–gel process. J. Nanopart. Res. 2012, 14, 792,  DOI: 10.1007/s11051-012-0792-x
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      Optical investigation of Tb3+-doped Y2O3 nanocrystals prepared by Pechini-type sol-gel process
      Back, M.; Massari, A.; Boffelli, M.; Gonella, F.; Riello, P.; Cristofori, D.; Ricco, R.; Enrichi, F.
      Journal of Nanoparticle Research (2012), 14 (4), 792/1-792/10CODEN: JNARFA; ISSN:1388-0764. (Springer)
      We report an optical study of Tb3+-doped Y2O3 nanocrystals synthesized by Pechini-type solgel method. The particles are investigated in terms of size and morphol. by means of X-ray diffraction and transmission electron microscopy anal. It is shown how the simple Pechini method allows for the growth of monocryst. nanoparticles with a vol.-weighted av. size of about 30 nm. The optical properties of Tb3+ in the host lattice are studied in terms of PL, PLE, and lifetimes. Moreover, a correlation between the type of decay curves, the emission and excitation bands' shapes, and the site location of the emitting Tb3+ in the host material Y2O3 is proposed.
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      Muenchausen, R. E.; Jacobsohn, L. G.; Bennett, B. L.; McKigney, E. A.; Smith, J. F.; Valdez, J. A.; Cooke, D. W. Effects of Tb doping on the photoluminescence of Y2O3:Tb nanophosphors. J. Lumin. 2007, 126, 838– 842,  DOI: 10.1016/j.jlumin.2006.12.004
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      Effects of Tb doping on the photoluminescence of Y2O3:Tb nanophosphors
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      Journal of Luminescence (2007), 126 (2), 838-842CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
      The effects of Tb doping on the photoluminescence (PL) of Y2O3:Tb nanophosphors were studied. Nanophosphors were prepd. by the glycine-nitrate soln. combustion technique using yttria and terbia powders as precursors. PL excitation spectra at room temp. consist of 2 overlapping bands centered at 277 and 304 nm, whereas emission spectra comprise several groups of lines corresponding to the 5D4→7FJ (J = 1-6) 4f electronic transitions of the Tb+3 ions. A direct comparison of nanophosphor and bulk concn.-quenching curves was obtained by annealing the nanophosphor powder and converting it to bulk material without altering the Tb concn. The peak in the nanophosphor concn.-quenching curve occurs at a concn. ∼3 times higher than that of the bulk.
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      Flores-Gonzalez, M. A.; Ledoux, G.; Roux, S.; Lebbou, K.; Perriat, P.; Tillement, O. Preparing nanometer scaled Tb-doped Y2O3 luminescent powders by the polyol method. J. Solid State Chem. 2005, 178, 989– 997,  DOI: 10.1016/j.jssc.2004.10.029
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      Preparing nanometer scaled Tb-doped Y2O3 luminescent powders by the polyol method
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      Journal of Solid State Chemistry (2005), 178 (4), 989-997CODEN: JSSCBI; ISSN:0022-4596. (Elsevier)
      Sub-micrometer Tb-doped Y2O3 luminescent powders were prepd. from nitrate precursors using the polyol method. Just after pptn., the powders consist of agglomerates with a spherical shape and a size ranging between 400 and 500 nm. Each agglomerate is composed of ultra-small crystallites (from 3 to 6 nm) of a bcc. oxide phase whose luminescence presents original features in comparison with bulk materials. Powders were further calcined at different temps. and for annealing <900°, highly cryst. samples with the classical green 5D4→7F5 luminescent transitions of Tb3+ ions are obtained. For optimized annealing temps., sintering between the agglomerates is avoided and a sub-micrometric powder with a narrow size distribution and a high luminescence is obtained.
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      Meng, Q.; Chen, B.; Xu, W.; Yang, Y.; Zhao, X.; Di, W.; Lu, S.; Wang, X.; Sun, J.; Cheng, L.; Yu, T.; Peng, Y. Size-dependent excitation spectra and energy transfer in Tb3+-doped Y2O3 nanocrystalline. J. Appl. Phys. 2007, 102, 093505– 093501,  DOI: 10.1063/1.2803502
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      Journal of Applied Physics (2007), 102 (9), 093505/1-093505/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Nanocrystal Y2O3 powders with different grain sizes and various doping concns. of Tb3+ were prepd. by an autocombustion reaction. The size and surface effects on the 4f-5d transitions and energy transfers between Tb3+ ions were studied by using x-ray diffraction, TEM, fluorescent spectra, and luminescent decay. The excitation spectra are composed of 2 parts: 1 is the contribution from the Tb3+ at/near the nanoparticle surfaces; another is from the Tb3+ inside the nanoparticles. The study on the concn. quenching and luminescent decay indicated that the energy transfers depopulating the 5D3 and 5D4 level were assigned to the mechanisms of elec. dipole-dipole and exchange interaction, resp. The size confinement greatly affects the energy transfer quenching the emission from the 5D3 level, but slightly affects the energy transfer quenching the emission from the 5D4 level.
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      Pavitra, E.; Raju, G. S. R.; Ko, Y. H.; Yu, J. S. A novel strategy for controllable emissions from Eu3+ or Sm3+ ions co-doped SrY2O4:Tb3+ phosphors. Phys. Chem. Chem. Phys. 2012, 14, 11296– 11307,  DOI: 10.1039/c2cp41722g
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      A novel strategy for controllable emissions from Eu3+ or Sm3+ ions co-doped SrY2O4:Tb3+ phosphors
      Pavitra, E.; Raju, G. Seeta Rama; Ko, Yeong Hwan; Yu, Jae Su
      Physical Chemistry Chemical Physics (2012), 14 (32), 11296-11307CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Trivalent rare-earth (RE) ions (Eu3+, Tb3+ and Sm3+) activated multicolor emitting SrY2O4 phosphors were synthesized by a sol-gel process. The structural and morphol. studies were performed by the measurements of X-ray diffraction profiles and scanning electron microscope (SEM) images. The pure phase of SrY2O4 appeared after annealing at 1300 °C and the doping of RE ions did not show any effect on the structural properties. From the SEM images, the closely packed particles were obsd. due to the roughness of each particle tip. The photoluminescence (PL) anal. of individual RE ions activated SrY2O4 phosphors exhibits excellent emission properties in their resp. regions. The Eu3+ co-activated SrY2O4:Tb3+ phosphor creates different emissions by controlling the energy transfer from Tb3+ to Eu3+ ions. Based on the excitation wavelengths, multiple (green, orange and white) emissions were obtained by Sm3+ ions co-activated with SrY2O4:Tb3+ phosphors. The decay measurements were carried out for analyzing the energy transfer efficiency and the possible ways of energy transfer from donor to acceptor. The cathodoluminescence properties of these phosphors show similar behavior as PL properties except the energy transfer process. The obtained results indicated that the energy transfer process was quite opposite to the PL properties. The calcd. CIE chromaticity coordinates of RE ions activated SrY2O4 phosphors confirmed the red, green, orange and white emissions.
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      Chukova, O.; Nedilko, S.; Titov, Y.; Sheludko, V. Crystallographic Features and Nature of Luminescence Centres of the Niobate and Tantalate Compounds with Layered Perovskite-Like Structure. Open Mater. Sci. J. 2018, 12, 2– 13,  DOI: 10.2174/1874088X01812010002
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      Cryslallographic features and nature of luminescence centres of the niobate and tantalate compounds with layered perovskite-like structure
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      Open Materials Science Journal (2018), 12 (), 2-13CODEN: OMSJAH; ISSN:1874-088X. (Bentham Open)
      Aim and Objectives The ultrafine powders of the perovskite-like layered AII3LaM3O12 (AII = Sr, Ba; M = Nb, Ta) compds. have been synthesized by heat treatment of co-pptd. hydroxy-carbonates. The luminescence of these compds. is reported for the first time. Methods: Luminescence spectra of all studied compds. are complex and contain two main wide bands with maxima near 2.9 and 2.5 eV. These luminescence bands were assigned to radiation electron transitions in the MoO67- mol. groups of different symmetry located in the various lattice positions. Conclusion: The energy levels scheme of the MoO67- group and related radiation and absorption transitions had been proposed.
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      Van Pieterson, L.; Heeroma, M.; de Heer, E.; Meijerink, A. Charge transfer luminescence of Yb3+. J. Lumin. 2000, 91, 177– 193,  DOI: 10.1016/S0022-2313(00)00214-3
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      A systematic study of charge transfer (CT) luminescence from Yb3+ is presented. CT luminescence was obsd. in LiYF4 (≈7 eV), aluminates, phosphates, oxides (≈ 4 eV) and oxysulfides (≈ 3 eV). In all cases the CT emission bands are broad and the Stokes shifts are large, ranging from 7000 cm-1 in oxysulfides and LiYF4 and up to 17,000 cm-1 in aluminates. The influence of the size of the host lattice cation site on the charge transfer luminescence was studied. A shift to longer wavelengths of the CT emission bands with increasing size of the cation site was obsd. Typical (radiative) decay times for the CT luminescence are 100 - 200 ns. The quenching of the Yb3+ CT luminescence in the various host lattices was studied and compared with the quenching of the Eu3+ emission under CT excitation in the same lattices. A good correlation was found. The quenching temps. of the Eu3+ emission are much higher than that of the Yb3+ CT luminescence. In lattices where Eu3+ emission quenches at very high temps. (= 500K) Yb3+ CT luminescence could be obsd.
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      A review. The state-of-art of Yb3+-doped inorg. materials will be presented due to strong interests in basics research and applied research. 4f13 configuration will be illustrated by few examples such as optical transition assignments in the IR range, diode pumped-IR laser, figure-of-merit of laser crystals, concn. quenching studies, evidence of Yb3+ pairs, the advance of Yb3+-doped cubic ceramics for laser sources, Yb3+ as sensitizer of activator ions and Yb3+ ion as structural probes of hosts. The last results analyzed will concern charge transfer processes of Yb3+ in oxides and the interests in luminescence.
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      Host lattice dependences of luminescence spectra and relaxation properties of Tb3+ are described. Reduced matrix elements relevant to the transitions were calcd. and compared with observations. The spectra and the decay times of the 5D4 → 7FJ transitions are in qual. agreement with the calcns. The elec.-dipole transition of 5D4 → 7F5 has the largest probability in the 5D4 → 7FJ emission transitions within the framework of the Judd-Ofelt approxn. When the crystal field acting on Tb3+ is very strong, the intensity of 5D4 → 7F3 can become comparable to that of 5D4 → 7F5. Magnetic-dipole transitions always have the largest probability for 5D4 → 7F5. Spontaneous multiphonon relaxation of 5D3 → 5D4 is sometimes a dominant decay mechanism of the 5D3 state. The obsd. multiphonon relaxation rates are ∼2 × 102 in YPO4, ∼8 × 103 in YBO3 and ∼2 × 104 in ScBO3 groups in units of sec-1.
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      The luminescence properties of Tb3+ in Tb3+-Tb3+ and Tb3+-Gd3+ majority-type pairs in CsCdBr3 are studied at 77 K and room temp. 5D3→5D4 cross-relaxation rates for Tb3+ in Tb3+-Tb3+ pairs are detd. The intrinsic 5D3→5D4 and 5D3→7FJ relaxation rates of Tb3+ in the majority-pair site are estd. The relative nos. of Tb3+-Tb3+ and Tb3+-Gd3+ pairs in Tb/Gd-doped CsCdBr3 are consistent with statistical pair formation.
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      Absorption and emission spectra are given for Yb3+-doped Y2O3, Lu2O3 and Gd2O3 at room temp. Y2O3 and Lu2O3 as close cubic matrixes, show Yb3+ similar spectra different of Yb3+ in Gd2O3 monoclinic structure. Here, the authors use a new method to study and optimize the main spectroscopic properties with only one concn. gradient sample. Finally, assignments of Yb3+ Stark levels and Raman vibrations in Y2O3, Lu2O3 and Gd2O3 single crystal are given.
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      Bharat, L. K.; Jeon, S. K.; Krishna, K. G.; Yu, J. S. Rare-earth free self-luminescent Ca2KZn2(VO4)3 phosphors for intense white light-emitting diodes. Sci. Rep. 2017, 7, 42348,  DOI: 10.1038/srep42348
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      Rare-earth free self-luminescent Ca2KZn2(VO4)3 phosphors for intense white light-emitting diodes
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      Scientific Reports (2017), 7 (), 42348CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      The com. available white-light-emitting diodes (WLEDs) are made with a combination of blue LEDs and yellow phosphors. These types of WLEDs lack certain properties which make them meagerly applicable for general illumination and flat panel displays. The soln. for such problem is to use near-UV (NUV) chips as an excitation source because of their high excitation efficiency and good spectral distribution. Therefore, there is an active search for new phosphor materials which can be effectively excited within the NUV wavelength range (350-420 nm). In this work, novel rare-earth free self-luminescent Ca2KZn2(VO4)3 phosphors were synthesized by a citrate assisted sol-gel method at low calcination temps. Optical properties, internal quantum efficiency and thermal stability as well as morphol. and crystal structure of Ca2KZn2(VO4)3 phosphors for their application to NUV-based WLEDs were studied. The crystal structure and phase formation were confirmed with XRD patterns and Rietveld refinement. The optical properties of these phosphor materials which can change the NUV excitation into visible yellow-green emissions were studied. The synthesized phosphors were then coated onto the surface of a NUV chip along with a blue phosphor (LiCaPO4:Eu2+) to get brighter WLEDs with a color rendering index of 94.8 and a correlated color temp. of 8549 K.
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      Boutinaud, P.; Cavalli, E.; Bettinelli, M. Emission quenching induced by intervalence charge transfer in Pr3+- or Tb3+-doped YNbO4 and CaNb2O6. J. Phys.: Condens. Matter 2007, 19, 386230,  DOI: 10.1088/0953-8984/19/38/386230
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      Emission quenching induced by intervalence charge transfer in Pr3+- or Tb3+-doped YNbO4 and CaNb2O6
      Boutinaud, P.; Cavalli, E.; Bettinelli, M.
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      The excitation and emission properties of Pr3+ and Tb3+ ions doped into YNbO4 (hereafter YNB) and CaNb2O6 (hereafter CNB) have been studied as a function of the temp. in the 10-600 K range. The obsd. quenching of the luminescence from the 3P0 (Pr3+) and 5D3 (Tb3+) states has been related to an electron transfer process from the trivalent rare earth to the Nb5+ ion inducing the formation of an intervalence charge transfer (IVCT) state. On this basis a detailed picture of the excited-state dynamics of the investigated compds. is presented, taking into account the characteristics of the host lattices and of the involved optically active ions.
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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02862.

    • Crystal field parameters and crystal field strength values, calculated energy level positions for Tb3+ and Yb3+ ion in InNbO4, InTaO4, and InVO4, oxygen position and relative charge for oxygen ligands in In site used to obtain the crystal field parameters, graphical representations of the In local environment, and Miller indices, d-space, dispersion angle, and simulated X-ray diffraction intensity of InVO4, InNbO4, and InTaO4 (PDF)


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