OOPS

A novel amino-benzothiadiazole bearing diphenylphosphine groups (L) was designed and synthesized. A number of its coordination compounds of Cu(I) (1·0.75C7H8, 2a,b), Pd(II) (4), and Pt(II) (5) were prepared demonstrating the coordination of L ligand via N and/or P atoms. The reaction with strong Lewis acid Zn(II) resulted in the rearrangement of P—N—P to P–P–N moiety and formation of complex 3. The structures of L, 2–5 were determined by single crystal X-ray (XRD) diffraction, while that of 1·0.75C7H8 was determined by powder XRD analysis. Intermolecular secondary bonding of S···S interactions in the compounds, unusual for benzothiadiazoles, was discussed supported by quantum chemical calculations. Differences in the structures of the compounds cause significant changes of photophysical properties. In particular, the position of the electronic absorption band is featured by the conformation of L, viz., the dihedral angle between the benzothiadiazole unit and the P—N—P moiety, as proved by TD-DFT calculations performed on model molecules. The photophysical properties of the compounds also strongly depend on the coordination mode of L: the presence of a bond between the metal and the N atom of the thiadiazole unit plays an essential role in defining the position of the absorption and emission bands as well as emission lifetime of these compounds. The thermally activated delayed fluorescence and phosphorescence mechanisms in a thermal equilibrium were determined for 1·0.75C7H8. Complex 5 features a broad emission band spanning the entire visible region; moreover, it shows an unusual clear-white luminescence, which remains visible even in the daylight.

We report the synthesis and the crystal and electronic structure as well as the optical and photocatalytic properties of novel photoactive materials of the general formula Bi26–xMgxO40. Two compounds with compositions of Bi24.28(3)Mg1.72(3)O40 and Bi24.05(3)Mg1.95(3)O40 are synthesized using the pyrolytic method. According to X-ray diffraction analysis, the materials are monocrystalline species. Their electronic bandgaps determined from Tauc plots are 2.41 eV [Bi24.28(3)Mg1.72(3)O40] and 2.69 eV [Bi24.05(3)Mg1.95(3)O40]. Keeping in mind that optical bandgaps are typically larger than their electronic counterparts, we find that the bismuthate bandgaps match well that of Bi24Mg2O40 (2.26 eV) predicted by density functional theory. Apparently, the synthesized bismuthates are indirect bandgap semiconductors just like Bi24Mg2O40. Both materials demonstrate nearly identical luminescence spectra. Their luminescence emission at 620 nm is most efficiently excited by 365 nm light. The materials’ photocatalytic properties are evaluated in a visible light-induced photocatalytic phenol degradation reaction. Rather low activity of both compounds is detected. However, Bi24.05(3)Mg1.95(3)O40 is ∼2 times more photocatalytically active than Bi24.28(3)Mg1.72(3)O40, which is associated with a higher Bi5+ content in the former.

We report the discovery of the zero thermal expansion (ZTE) effect in BPO4, a famous deep-ultraviolet (DUV) optical material with cristobalite-like structure. It is revealed that BPO4 has a linear ZTE coefficient of −0.16(5) MK–1 along the c-axis as temperature increases from 13 to 300 K, which originates from the subtle counterbalance between the rotation-induced expansion and contraction effects among BO4 and PO4 groups. BPO4 is a unique DUV cristobalite-like material exhibiting the linear ZTE behavior.

Local structure modification in solid solution is an essential part of photoluminescence tuning of rare earth doped solid state phosphors. Herein we report a new solid solution phosphor of Eu2+-doped xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7 (0 ≤ x ≤ 1), which share the same β-Ca3(PO4)2 type structure in the full composition range. Depending on the x parameter variation in xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7:Eu2+, the vacancies generated in the M(4) site enable the nonlinear variation of cell parameters and volume, and this increases the magnitude of M(4)O6 polyhedra distortion. The local structure modulation around the Eu2+ ions causes different luminescent behaviors of the two-peak emission and induces the photoluminescence tuning. The shift of the emission peaks in the solid solution phosphors with different compositions has been discussed. It remains invariable at x ≤ 0.5, but the red-shift is observed at x > 0.5 which is attributed to combined effect of the crystal field splitting, Stokes shift, and energy transfer between Eu2+ ions. The temperature-dependent luminescence measurements are also performed, and it is shown that the photoionization process is responsible for the quenching effect.

The union of temperature-dependent phase transition and relating structural transformation via modification of chemical compositions is of fundamental importance for the discovery of new materials or their functional properties optimization. Herein, the synthesis temperature and Eu2+-doping content induced phase selection and variations of the local structures in nepheline, low-carnegieite and high-carnegieite types of NaAlSiO4 polymorphs were studied in detail. The luminescence of Eu2+ in low-carnegieite and nepheline phases shows blue (460 nm) and yellow (540 nm) broad-band emissions, respectively, under near-ultraviolet excitation. The photoluminescence evolution can be triggered by the different synthesis temperatures in relation to the Eu2+-doping concentration, as corroborated by density functional theory calculations on the local coordination structures and corresponding mechanical stabilities in terms of the Debye temperature. The fabricated white light-emitting diode device with high color rendering index demonstrates that the multicolor phosphors from one system provides a new gateway for the photoluminescence tuning.

Eu2+ local environments in various crystallographic sites enable the different distributions of the emission and excitation energies and then realize the photoluminescence tuning of the Eu2+ doped solid state phosphors. Herein we report the Eu2+-doped Ca10M(PO4)7 (M = Li, Na, and K) phosphors with β-Ca3(PO4)2-type structure, in which there are five cation crystallographic sites, and the phosphors show a color tuning from bluish-violet to blue and yellow with the variation of M ions. The difference in decay rate monitored at selected wavelengths is related to multiple luminescent centers in Ca10M(PO4)7:Eu2+, and the occupied rates of Eu2+ in Ca(1), Ca(2), Ca(3), Na(4), and Ca(5) sites from Rietveld refinements using synchrotron power diffraction data confirm that Eu2+ enters into four cation sites except for Ca(5). Since the average bond lengths d(Ca–O) remain invariable in the Ca10M(PO4)7:Eu2+, the drastic changes of bond lengths d(M–O) and Eu2+ emission depending on the variation from Li to Na and K can provide insight into the distribution of Eu2+ ions. It is found that the emission band at 410 nm is ascribed to the occupation of Eu2+ in the Ca(1), Ca(2), and Ca(3) sites with similar local environments, while the long-wavelength band (466 or 511 nm) is attributed to Eu2+ at the M(4) site (M = Na and K). We show that the crystal-site engineering approach discussed herein can be applied to probe the luminescence of the dopants and provide a new method for photoluminescence tuning.

Discovery of new semiconductor candidates with suitable band gaps is a challenge for optoelectronic application. A facile solvothermal synthesis of a new ternary chalcogenide semiconductor CsCu5Se3 is reported. The telluride CsCu5Te3 is also predicted to be stable. CsCu5Se3 is isostructural with CsCu5S3 (space group Pmma). The band gap calculations of these chalcogenide semiconductors using hybrid density functional theory indicate nearly direct band gaps, and their values (about 1.4 eV) were confirmed by the optical absorption spectroscopy. These alkali metal copper chalcogenides are interesting examples of copper-rich structures which are commonly associated with favorable photovoltaic application.

Eu2+ and Ce3+/Li+ singly doped and Eu2+/Ce3+/Li+-codoped Ca1.65Sr0.35SiO4 phosphors have been synthesized by a solid-state reaction method. The crystal structure was determined by Rietveld refinement to verify the formation of the αL′-Ca2SiO4 phase with the Sr addition into Ca2SiO4, and the preferred crystallographic positions of the Eu2+ and Ce3+/Li+ ions in Ca1.65Sr0.35SiO4 were analyzed based on a comparison of the unit-cell volumes and the designed chemical compositions of undoped isostructural compounds Ca(2–x)SrxSiO4 (x = 0.25, 0.35, 0.45, 0.55 and 0.65). Ce3+/Li+ singly activated Ca1.65Sr0.35SiO4 phosphors exhibit strong absorption in the range of 250–450 nm and a blue emission peak centered at about 465 nm. When Eu2+ ions are codoped, the emission colors of Ca1.65Sr0.35SiO4:Ce3+,Li+,Eu2+ phosphors under the irradiation of 365 nm can be finely tuned from blue to green through the energy transfer from Ce3+ to Eu2+. The involved energy-transfer process between Ce3+ and Eu2+ and the corresponding mechanism are discussed in detail. The reported Ca1.65Sr0.35SiO4:Ce3+,Li+,Eu2+ phosphor might be a candidate for color-tunable blue-green components in the fabrication of near-ultraviolet-pumped white-light-emitting diodes (WLEDs).

The orthosilicate phosphors demonstrate great potential in the field of solid-state lighting, and the understanding of the structure–property relationships depending on their versatile polymorphs and chemical compositions is highly desirable. Here we report the structural phase transformation of Ca2–xSrxSiO4:Ce3+ phosphor by Sr2+ substituting for Ca2+ within 0 ≤ x < 2. The crystal structures of Ca2–xSrxSiO4:Ce3+ are divided into two groups, namely, β phase (0 ≤ x < 0.15) and α′ phase (0.18 ≤ x < 2), and the phase transition (β → α′) mechanism originated from the controlled chemical compositions is revealed. Our findings verified that the phase transition Pnma (α′-phase) ↔ P21/n (β-phase) can be ascribed to the second-order type, and Sr2+ ions in Ca2–xSrxSiO4 preferentially occupy the seven-coordinated Ca2+ sites rather than the eight-coordinated sites with increasing Sr2+ content, which was reflected from the Rietveld refinements and further clarified through the difference of the Ca–O bond length in the two polymorphs of Ca2SiO4. The emission peaks of Ce3+ shift from 417 to 433 nm in the composition range of 0 ≤ x ≤ 0.8, and the difference in the decay curves can also verify the phase transformation process. Thermal quenching properties of selected Ca2–xSrxSiO4:Ce3+ samples were evaluated, and the results show that the integral emission intensities at 200 °C maintain >90% of that at room temperature suggesting superior properties for the application as white light-emitting diodes (w-LEDs) phosphors.

Hydrothermally reacting Lu(NO)3 and Na2WO4·2H2O at 200 °C and pH = 8 produced the new compound NaLuW2O8·2H2O, which was analyzed via the Rietveld technique to crystallize in the orthorhombic system (space group: Cmmm) with cell parameters a = 21.655(1), b = 5.1352(3), and c = 3.6320(2) Å and cell volume V = 403.87(4) Å3. The crystal structure presents −(NaO6)-(NaO6)– and −(LuO4(H2O)2WO5)-(LuO4(H2O)2WO5)– alternating layers linked together by the O2– ion common to NaO6 octahedron and WO5 triangle bipyramid. Tetragonal structured and phase-pure Na(Lu0.87Ln0.03Yb0.1)(WO4)2 phosphors (Ln = Ho, Er, and Tm) were directly produced by calcining their NaLuW2O8·2H2O analogous precursors at 600 °C for 2 h, followed by a detailed study of their downconversion/upconversion (DC/UC) photoluminescence. It was shown that the UC luminescence is dominated by a red band at ∼650 nm for Ho3+ (5F5 → 5I8 transition), green bands at ∼500–575 nm for Er3+ (2H11/2/4S3/2 → 4I15/2 transitions) and a blue band at ∼476 nm for Tm3+ (1G4 → 3H6 transition), all via a three-photon process. DC luminescence of the phosphors is characterized by a ∼545 nm green emission for Ho3+ (5F4/5S2 → 5I8 transition, λex = 453 nm), ∼500–575 nm green emissions for Er3+ (2H11/2/4S3/2 → 4I15/2 transitions, λex = 380 nm), and a ∼455 nm blue emission for Tm3+ (1D2 → 3F4 transition, λex = 360 nm), with CIE chromaticity coordinates of around (0.27, 0.71), (0.26, 0.72), and (0.15, 0.04), respectively.

Exploring new perovskite-related solid-state materials and the investigating composition-dependent structural and physical properties are highly important for advanced functional material development. Herein, we present the successful hydrothermal synthesis of tetragonal CsPb2Cl5 and the anion-exchange phase formation of CsPb2(Cl1–xBrx)5 (x = 0–1) solid solutions. The CsPb2(Cl1–xBrx)5 crystal structures, which crystallize in the tetragonal system, space group I4/mcm, with parameters similar to those of CsPb2Cl5, have been determined by Rietveld analysis. The optical band gap was obtained by UV–vis spectroscopy, and the band structure was further calculated by the full-potential method within the generalized gradient approximation. It was revealed that the band gap in CsPb2(Cl1–xBrx)5 solid solutions can be tuned over the range of 4.5–3.8 eV by anion substitution.

Silver–europium double sulfate AgEu(SO4)2 was obtained by solid-phase reaction between Ag2SO4 and Eu2(SO4)3. The crystal structure of AgEu(SO4)2 was determined by Monte Carlo method with simulated annealing, and after that, it was refined by the Rietveld method from X-ray powder diffraction data. The compound crystallizes in the triclinic symmetry, space group P1̅ (a = 0.632929(4), b = 0.690705(4), c = 0.705467(4) nm, α = 98.9614(4), β = 84.5501(4), γ = 88.8201(4)°, V = 0.303069(3) nm3). Two types of sulfate tetrahedra were found in the structure, which significantly affects the spectroscopic properties in the IR-range. In the temperature range of 143–703 K, the average linear thermal expansion coefficients of cell parameters a, b, and c are very similar, (1.11–1.67) × 10–5 K–1 in magnitude, and therefore, AgEu(SO4)2 expands almost isotropically. Upon heating in argon flow, AgEu(SO4)2 is stable up to 1053 K. The luminescence spectra in the region of ultranarrow 5D0–7F0 transition contain a single narrow and symmetric line at 579.5 nm that is evidence of good crystalline quality of AgEu(SO4)2 and uniform local environment of Eu3+ ions in the structure. Distribution of luminescence bands is determined by the environment of Eu3+ ions in the structure. Influence of Ag+ ions on the electron density distribution at Eu sites is detected.

Hydrothermal reaction of La(NO3)3 and Na2WO4·2H2O at 100 °C and pH 8 resulted in the formation of new compound NaLaW2O7(OH)2(H2O), as confirmed by the X-ray diffraction results, chemical composition, Fourier transform infrared, thermogravimetric/differential thermal analysis, and transmission electron microscopy analyses. The crystal structure was determined in the triclinic system (space group P1̅), with lattice constants a = 5.8671(2) Å, b = 8.2440(2) Å, and c = 9.0108(3) Å, axis angles α = 93.121(2)°, β = 75.280(2)°, and γ = 94.379(2)°, and cell volume V = 420.03(2) Å3. The structure contains two-dimensional layers of -(W1O6)-(W1O6)-(W2O6)-(W2O6)-(W1O6)-(W1O6)- and -LaO9-LaO9- chains alternating in the a–b plane, which are linked together through NaO6 octahedral trigonal prisms by edges to form a three-dimensional net. Dehydration of the compound proceeds up to a low temperature of ∼350 °C and results in the formation of technologically important NaLa(WO4)2 double tungstate, which is thus a unique precursor for the latter. Na(La,RE)W2O7(OH)2(H2O) and Na(La,RE)(WO4)2 solid solutions separately doped with the practically important activators for which RE = Eu, Tb, Sm, and Dy were also successfully synthesized and investigated for their structural features and photoluminescence properties, including excitation, emission, quantum yield, emission color, and fluorescence decay kinetics. The compounds were shown to exhibit dominantly strong red (∼616 nm for Eu3+; λex = 395 or 464 nm), green (∼545 nm for Tb3+; λex = 278 or 258 nm), deep red (∼645 nm for Sm3+; λex = 251 nm), and yellow (∼573 nm for Dy3+; λex = 254 nm) emission upon being irradiated with the peak wavelengths of their strongest excitation bands.

Organic–inorganic hybrid metal halides with zero-dimensional (0D) structure has emerged as a new class of light-emitting materials. Herein, a new lead-free compound (C9NH20)2MnBr4 has been discovered and a temperature-dependent phase transition has been identified for two phases (space group P21/c and C2/c) in which individual [MnBr4]2– anions connect with organic cations, (C9NH20+) (1-buty-1-methylpyrrolidinium+), forming periodic structure with 0D blocks. A green emission band, peaking at 528 nm with a high photoluminescence quantum efficiency (PLQE) of 81.08%, has been observed at room temperature, which is originated from the 4T1(G) to 6A1 transition of tetrahedrally coordinated Mn2+ ions, as also elaborated by density functional theory calculation. Accordingly, a fast, switchable, and highly selective fluorescent sensor platform for different organic solvents based on the luminescence of (C9NH20)2MnBr4 has been developed. We believe that the hybrid metal halides with high PLQE and the exploration of these as a fluorescence sensor will expand the applications scope of bulk 0D materials for future development.

The equivalent or heterovalent substitution strategy is an efficient way to stimulate photoluminescence tuning or to optimize the luminescence performances of phosphor materials. Garnet-type compounds receive much attention as phosphor hosts because of their flexible structural frameworks. Herein, a garnet-type Lu2MgAl4SiO12:Eu2+ phosphor with broad-band blue-green emission is first explored with two-site occupation by varying the Eu2+ content. Two host-substitution approaches to controlling the luminescence behavior of Lu2MgAl4SiO12:Eu2+ phosphor are implemented. The cation substitution strategy of Ca2+ for Mg2+ achieves tunable emission from 463 to 503 nm together with broadening emission bands in Lu2Mg1–yCayAl4SiO12:Eu2+ phosphors. Moreover, chemical unit cosubstitution of [Ca2+–Ge4+] replacing [Lu3+–Al3+] results in Lu2–zCazMgAl4–zGezSiO12:Eu2+ phosphors, which induce a red shift of the emission peak of about 60 nm and a broadening in the emission spectra with increasing Ca2+ and Ge4+ concentrations. The possible photoluminescence tuning mechanism is ascribed to the coordination sphere variation in the EuO8 polyhedron depending on the changing neighboring cations. The proposed approaches on equivalent or heterovalent substitution can contribute to the development of Eu2+-activated garnet-type phosphors with regulation of the luminescence performance and further initiate research discovering new phosphors for white-light-emitting diodes.

A series of iso-structural La5(Si2+xB1–x)(O13–xNx):Ce3+ phosphors with apatite structure have been prepared. A combination of powder X-ray diffraction and neutron scattering technique was employed to explore the crystal structural evolution and the rigid nature from oxy- to oxynitride-based apatites, and some local structures were also characterized by HRTEM and 29Si NMR data, respectively. The new La5(Si2+xB1–x)(O13–xNx):Ce3+ solid solution phosphors gave continuously controlled emission from 421 nm [La5Si2BO13:Ce3+, end-member (x = 0)] to 463 nm (La5Si3O12N:Ce3+, end-member (x = 1)). Substitution of B3+ and O2– by Si4+ and N3– in La5(Si2+xB1–x)(O13–xNx):Ce3+ phosphors produced more covalency into the crystal field environment around the Ce3+ ions inducing the red-shifted emission, further improving the thermal stability of the oxynitride-based apatite phosphors. The proposed approach from oxy- to oxynitride based iso-structural phases could significantly contribute to future research in designing complex solid solution phosphors.

The powder α-Eu2(MoO4)3 sample was prepared by the solid-state reaction method. The phase purity of the final powder product was verified by X-ray diffraction analysis. The constituent element core levels and valence band are measured by X-ray photoelectron spectroscopy as a function of Ar+ ion (2.5 keV, 7–8 μA/cm2) bombardment time. The formation of Mo5+ and Mo4+ states at high bombardment times was detected. The Eu–O and Mo–O bonding was considered in comparison with other Eu3+- and Mo6+-containing oxides using binding energy difference parameters. The transparency range obtained for the pure α-Eu2(MoO4)3 tablet is λ = 0.41–0.97 μm, as estimated at the transmission level of 5%. The short-wavelength cut edge in α-Eu2(MoO4)3 is governed by the direct allowed optical transitions within the band gap of Eg = 3.74 eV (300 K). The band structure of α-Eu2(MoO4)3 was calculated by ab initio methods and strongly different results were obtained for the spin up/down configurations. The Eu-4f states are located around 2.2 eV and −4.0 eV for spin up (↑) and the structures situated at around 6.5 and 5.5 eV for spin down (↓) configuration. The calculated spin magnetic moments are in excellent relation to the Slater-Pauling rule and within the Eu sphere the magnetic moment of 4f electrons is ∼5.99 μB.

Doping is able to create novel optoelectronic properties of halide perovskites, and the involved mechanism of efficient emission is still a challenge. Herein Mn2+ substitution into 2D layered perovskites (C8H20N2)PbBr4 was investigated, demonstrating broad-band orange-red emission originating from the 4T1 → 6A1 transition of Mn2+ dopant. The photoluminescence quantum yield (PLQY) of Mn2+ emission is up to 60.8% related to the energy transfer in coupled states. We verify that an actual Mn2+ dopant as low as 0.476% reaches a high PLQY, whereas the nominal adding amount is 0.8 as the Mn2+/Pb2+ ratio. The small activation energy (∼6.72 meV) between the Mn2+ d state and the trap state accounts for this highly efficient energy transfer and photoluminescence. The proposed luminescence mechanism in Mn2+-doped 2D halide perovskites would provide unique insights into the doping design toward high-performance luminescence materials.