Impact of Structural Changes on Energy Transfer in the Anion-Engineered Re3+:Y2O3 Through Low-Temperature Synthesis Approach

Anion engineering has proven to be an effective strategy to tailor the physical and chemical properties of metal oxides by modifying their existing crystal structures. In this work, a low-temperature synthesis for rare earth (RE)-doped Y2O2SO4 and Y2O2S was developed via annealing of Y(OH)3 intermediates in the presence of elemental sulfur in a sealed tube, followed by a controlled reduction step. The crystal structure patterns (X-ray diffraction) and optical spectra (UV-IR) of Y2O2SO4, Y2O2S, and crystalline Y2O3 were collected throughout the treatment steps to correlate the structural transformations (via thermogravimetric analysis) with the optical properties. Local and long-range crystallinities were characterized by using X-ray and optical spectroscopy approaches. Systematic shifts in the Eu3+ excitation and emission peaks were observed as a function of SO42– and S2– concentrations resulting from a crystal evolution from cubic (Y2O3) to trigonal (Y2O2S) and monoclinic (Y2O2SO4), which can modify the local hybridization of sensitizer dopants (i.e., Ce3+). Ultimately, Tb3+ and Tb3+/Ce3+ doping was employed in these hosts (Y2O2SO4, Y2O2S, and Y2O3) to understand energy transfer between sensitizer and activator ions, which showed significant enhancement for the monoclinic sulfate structure.


INTRODUCTION
White light-emitting diodes (w-LEDs) have substantially replaced conventional light sources (incandescent and fluorescent lamps) owing to their outstanding features such as low energy consumption, 1 high brightness, 2 high luminous efficiency, 3 and long operational lifetimes. 4Rare earths (RE 3+ )doped phosphors for w-LEDs have garnered considerable attention due to their widespread applications in the field of lighting. 5,6−9 However, Tb 3+ -doped materials suffer from a lower luminescence due to the weak and narrow absorption cross-section (σ abs ∼ 10 −20 cm 2 ) originating from spin-forbidden electric dipole transitions between 4f energy levels of Tb 3+ . 10,11−15 For instance, GdPO 4 :Ce 3+ /Tb 3+ nanorods have been reported to be efficient hosts for the energy transfer from Ce 3+ to Tb 3+ ions with 28% quantum efficiency. 16The study revealed that the energy transfer depends upon the oxidation state of Ce ions in the host since dispersing codoped (GdPO 4 :Ce 3+ /Tb 3+ ) sample into KMnO 4 (oxidizing agent) solution resulted in significant luminescence quenching due to a change in the oxidation state of Ce from Ce 3+ to Ce 4+ .Alternatively, core−shell Y 2 Sn 2 O 7 :Tb 3+ /Ce 3+ @SiO 2 has been successfully synthesized to enhance poor energy transfer from Ce 3+ to Tb 3+ which stems from D 3d centrosymmetric lanthanide sites in Y 2 Sn 2 O 7 and vibrational relaxation of surface hydroxyl groups on the nanoparticles. 17An improvement in the energy transfer from Ce 3+ to Tb 3+ is primarily associated with the migration of Ce 3+ and Tb 3+ ions from centrosymmetric lanthanide sites to the interface between silica and core Y 2 Sn 2 O 7 nanoparticles.Nagashima et al. investigated the energy transfer between Ce 3+ and Tb 3+ in the phase-pure Ce 3+ /Tb 3+ -codoped LaLuO 3 by selectively doping Ce 3+ /Tb 3+ at La (A) and Lu (B) sites. 18he energy transfer was maximized when Tb 3+ was doped in A sites.In contrast, another study showed that Tb 3+ /Ce 3+ codoping in cubic phase Y 2 Zr 2 O 7 can exhibit the luminescence quenching instead of sensitization due to the more symmetric local environment of Ce 3+ -doped sites in the host matrix and potential oxidation to Ce 4+ . 19−22 Over the past decade, RE-doped oxysulfates and oxysulfides have been more effective host matrices than oxides for enhancing luminescent properties. 23,24Various high-temperature synthesis methods have been developed to prepare RE oxysulfates and oxysulfides.For instance, Eu 3+ -doped Y 2 O 2 SO 4 nanoparticles have been synthesized through electrospinning and combustion at 1000 °C with sulfur powder in the air. 25 2 O 2 SO 4 :Eu 3+ was also synthesized successfully at 800 °C via a coprecipitation method employing ammonium sulfate ((NH 4 ) 2 SO 4 ) as a sulfur source. 26The charge transfer peak in the excitation and 5 D 0 → 7 F 2 transition in the emission spectrum of Eu 3+ shifted to higher wavelengths (617−618 nm) compared with Y 2 O 3 :Eu 3+ .Low-temperature approaches for Y 2 O 2 SO 4 :Eu 3+ have been reported 27,28 ; however, the pure phase Y 2 O 2 SO 4 is only obtained for temperatures above 950 °C.Similarly, hexagonal (trigonal) phase Yb 3+ /Er 3+ doped Y 2 O 2 S was obtained through solid-state flux fusion with sulfur powder at 1150 °C in 1 h with internal quantum yields of green emission and infrared emission of 67 and 97%, respectively. 29The incorporation of S 2− redshifts the charge transfer band (CTB) to approximately 325 nm allowing for lower energy excitation sources. 30The need for hightemperature postprocessing for sulfur incorporation, release of hazardous gases, and utilization of toxic precursors (such as CS 2 ) can limit the application of these materials.Thus, it is critical to develop new synthesis routes to fabricate those materials.
This study employed a low-temperature two-step facile hydrothermal/combustion process with sulfur powder to synthesize Y 2 O 2 SO 4 :RE 3+ (Eu 3+ , Tb 3+ , Tb 3+ /Ce 3+  31,32 and highlighted in Figure 1. 33 ) and sulfur powder.For example, 200 mg of intermediate was mixed with 200 mg of excess sulfur powder (Alfa Aesar, 99.5%) and transferred into a quartz tube (20 cm long, 1 mm diameter, and 2 mm thick).Both ends of the quartz tube were sealed with a propane torch to avoid sulfur evaporation in the furnace.The sealed tube was annealed at 500 °C for 4 h with a 10 °C/min ramp rate in a box furnace.The obtained sample was washed with deionized water and toluene several times and dried at 130 °C.The obtained oxysulfate samples were further reduced to Y 2 O 2 S with 5% H 2 /95% N 2 flow.Y 2 O 2 SO 4 samples with various dopants were placed in a ceramic crucible and transferred to the tube furnace.Before starting the reduction, the tube furnace was purged with 5% H 2 /95% N 2 for 30 min and ramped to 600 °C at 10 °C/min and the dwell time was 2 h.

Characterization.
The crystal structures were obtained by using powder XRD with a PANalytical XRD operating at 45 kV and 40 mA and using a Cu Kα (λ = 1.54 Å) radiation source from 5 to 70°2θ scan range with a step size of 0.01°.Rietveld refinement was performed on the observed XRD patterns using the General Structure Analysis System II (GSAS II) software. 34The following parameters were refined: histogram scale factor, displacement, lattice parameters, microstrain, crystallite size, instrument parameters, background, atomic coordinates, and thermal parameters.An Edinburgh FLS1000 PL spectrometer equipped with a PMT detector and a 450 W ozone-free xenon arc lamp as a light source was employed to perform the photoluminescence and lifetime measurements.A quartz spectrophotometer cell (Starna Cells, Inc.) was used to place the powder samples to obtain the room-temperature emission and excitation spectra.The PL measurements were collected with a bandwidth of 1.5 nm, a dwell time of 0.4 s, and a step size of 1 nm in the ranges of 200−400 nm (excitation) and 470−700 nm (emission), respectively.A 455 nm filter was employed to reduce the fluorescence from the lamp for emission measurements.The The Journal of Physical Chemistry C lifetime measurements were obtained at 77 K with a microsecond flash lamp (frequency: 25 kHz, 1−2 μs pulse) over a range of 10 ms with a 2 ms delay time, resulting in a 5 μs detector response.To quantify lifetime values for all hosts, decay curves of all samples were fitted with an exponential formula: where I and t are the luminescence intensity and time (μs), respectively, A is the fitting constant, and τ is the lifetime.Attenuated total reflectance Fourier transform infrared (ATR-FTIR) measurements were carried out in a Thermo Scientific Nicolet 6700 FTIR instrument equipped with a Smart iTR sampling accessory (Thermo Scientific).A 2−3 mg sample was mixed and ground with 100 mg of KBr (EMD chemicals, ACS grade) and was placed on the crystal plate and pressed.All spectra were collected in a scan range from 1400 to 500 cm −1 with a 4 cm −1 spectral resolution by averaging 128 scans.Inductively coupled plasma-optical emission spectroscopy (ICP-OES) measurements were performed using a PerkinElmer Optima 8000.All samples were prepared by digesting 20 mg of the Y 2 O 3 :Tb 3+ (5 mol %), Ce 3+ (5 mol %), Y 2 O 2 S:Tb 3+ (5 mol %), Ce 3+ (5 mol %), Y 2 O 2 SO 4 :Tb 3+ (5 mol %), and Ce 3+ (5 mol %) in an aqueous HNO 3 (MiliporeSigma, 65%) and HCl (VWR BDH Chemicals, 38%).For the measurement, the digested samples were diluted to 12−15 ppm by using 2% HNO 3 .Meanwhile, Y, Tb, Ce, and S standards (1000 ppm, Inorganic Ventures) were diluted to 1, 5, 10, and 20 ppm with 2% HNO 3 for the calibration.According to the ICP-OES measurements, the actual concentrations of Tb 3+ and Ce 3+ were 3 mol % for this sample, as shown in Table S2.TGA/ DSC was performed using a TA SDT Q600.About 18 mg of Y 2 O 2 SO 4 was loaded to a sample holder and ramped at 5 °C/ min to 800 °C with a final dwell time of 2 h.For reducing these samples, a 5% H 2 /95% N 2 flow was used.

RESULTS AND DISCUSSION
and Y 2 O 2 S were synthesized and denoted as S1, S2, and S3, respectively.The XRD patterns of the samples are shown in Figure 2a.For clarity, the XRD pattern of the S2 was shown with a magnified view, specifically highlighting the angle range between 28.5 and 32.5°, in the Supporting Information (Figure S1).All samples were obtained from the same hydrothermal synthesis method 32,35 and crystallite sizes, based on the Scherrer equation, for S1, S2, and S3 were found to be roughly 20.62, 22.59, and 21.86 nm, respectively.This suggests that the chemical transformation does not necessarily impact the crystallite sizes.Sample S1 matched well with the cubic phase Y 2 O 3 with space group Ia3̅ (JCPDS card No.04-016-1857). 36Since the sulfur evaporates at low temperatures (around 444 °C) at standard atmospheric pressure, 37 the air flow in the tube furnace is believed to remove sulfur vapors quickly, resulting in precursor dehydration to form Y 2 O 3 .Nevertheless, when the precursor and sulfur mixture were reacted in the sealed quartz tube with air, a new diffraction ) ions.No other impurity phases were observed.Furthermore, the reduced S2 samples resulted in a secondary transformation in the diffraction pattern due to the formation of S3.This transformation is attributed to the presence of Y 2 O 2 S. Minor peaks are observed around 29.1°corresponding with Y 2 O 3 impurities due to overreduction of the samples.The crystal structure of sample S3 agrees well with that of the trigonal phase Y 2 O 2 S (space group: P3̅ m1, JCPDS card No.00-024-1424). 38All three diffraction patterns and their refinements are shown in the Supporting Information (Figure S2), including the associated peak positions, to confirm the phase purity of the assynthesized materials.Y 2 O 3 was refined with the cubic phase Y 2 O 3 , 36 while Y 2 O 2 S and Y 2 O 2 SO 4 were refined from the trigonal phase Y 2 O 2 S 39 and the monoclinic phase Eu 2 O 2 SO 4 , 40 respectively.Atomic positions and isotropic displacement parameters for all samples (U iso ) are given in the Supporting Information (Table S1).The refinement results indicate that the synthesis methods are possible low-temperature routes to obtain relatively defect-free nanostructures.The lattice parameters and statistical factors of all samples confirm the reasonable fitting of the experimental data (Table 1).
As further confirmation, FTIR spectroscopy was used to probe the structural and vibrational frequencies of the synthesized crystals, as shown in Figure 2b, specifically focusing on vibrations stemming from sulfur bonds.For sample S1, the strong absorption band located at 560 cm −1 is associated with Y−O lattice vibrations which confirm the formation of crystalline phase Y 2 O 3 nanostructures. 41In sample S2, the peaks located at 1061, 1126, and 1216 cm −1 are attributed to asymmetric ν 3 (SO 4 2− ) stretches. 42The peak centered at around 1003 cm −1 is due to ν 1 (SO 4 2− ), and peaks at 608, 621, and 666 cm −1 were assigned as the ν 4 (SO 4

2−
). 43 The appearance of strong triply degenerate ν 3 and ν 4 modes indicates that the point symmetry of the sulfate group is distorted from T d to C 2ν . 44Such a distortion shows that the coordination between SO 4 2− and Y 3+ is the bridged bidentatetype, in which two oxygen atoms from the sulfate group are directly coordinated with Y 3+ and the rest of them are uncoordinated.The single vibrational mode around 1003 cm −1 (ν 1 ) is usually IR-inactive, but it was observed in FTIR spectra due to the low symmetry in the oxysulfate. 45Moreover, after the annealing of Y 2 O 2 SO 4 in the hydrogen environment, the splits around 1100 cm −1 became narrower and blue-shifted approximately 14 cm −1 , and the splits between 608 and 670 cm −1 disappeared, resulting from the reduction of SO 4 2− groups to S 2− ions. 26This is due to the trigonal crystal structure of Y 2 O 2 S where S 2− is directly bonding with Y 3+ and forms a stronger bond, which increases the frequency.
To further understand the local crystal structure, the samples were doped with 5 mol % Eu 3+ to take advantage of the crystal structure-dependent luminescence associated with the electric (∼615 nm) and magnetic dipole (590 nm) transitions. 46,47ow RE-dopant concentrations (up to 15 mol %) have been shown to have little to no impact on the crystal structures due to the similarities of the RE 3+ and Y 3+ ionic radii. 35,48hotoluminescent excitation and emission spectra were collected at room temperature for all three samples (Figure 2c).The sample S1 exhibits a strong, broad excitation band with a maximum value at 248 nm owing to the CTB between O 2− and Eu 3+ . 49With the incorporation of sulfate ions, the CTB maximum redshifts to 267 nm for the S2.A reduction of this sample to Y 2 O 2 S:Eu 3+ (5 mol %, S3) resulted in an appearance of an O 2− -Eu 3+ band at approximately 260 nm, with additional broadband peaks between 300 and 375 nm, indicative of a 3p (S 2− ) to 4f (Eu 3+ ) CTB. 50 The CTB redshift is attributed to the different electronegativities, specifically 3.44 (O) and 2.6 (S), resulting in weaker Eu−O bonding. 51For sample S3, the shift in excitation peak around 260 nm has been reported to be a function of both the O 2− -Eu 3+ hybridization and the Y 2 O 2 S adsorption edge. 52Furthermore, the broad CTB between 300 and 375 nm is due to S 2− -Eu 3+ hybridization, and is consistent with the crystal structure of Y 2 O 2 S where each Eu 3+ is directly coordinated with three S 2− .Jorgensen proposed an empirical formula to calculate energy of charge transfer between anion and metal ions given by where χ opt (x) and χ opt (m) are the optical electronegativities of the anion (x) and metal (m). 53The calculated CTB energies of Y 2 O 3 :Eu 3+ , Y 2 O 2 SO 4 :Eu 3+ , and Y 2 O 2 S:Eu 3+ were reported by Dorenbos as 5.12, 4.59, and 3.61 eV, respectively. 54Thus, by using an equation of λ = 1240/E CTB , the CTB wavelengths can be estimated as 242, 270, and 343 nm for Y 2 O 3 :Eu 3+ , Y 2 O 2 SO 4 :Eu 3+ , and Y 2 O 2 S:Eu 3+ , respectively, agreeing well with the above results. 55Moreover, the emission spectra of all three samples were obtained under 395 nm excitation.The predominant red emission was observed at 611 nm for the S1, and this is attributed to the electric dipole transitions 5 D 0 → 7 F 2 .The electric dipole transitions redshifted to 617 and 626 nm for samples S2 and S3, respectively.Figure 2d highlights the correlation between the S/O stoichiometric mole ratios and the wavelength of the electric dipole transition 5 D 0 → 7 F 2 at the peak maximum and the asymmetric ratio (R).The R value is defined as the intensity ratio of 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 , which describes the electric and magnetic dipole transitions, the former of which is known to be symmetry-  57 Nevertheless, the R ratios for the monoclinic Y 2 O 2 SO 4 (8.9) and trigonal Y 2 O 2 S (8.4) are lower than the Y 2 O 3 .This is attributed to the incorporation of SO 4 2− and S 2− ,which reduced the asymmetry of Eu 3+ in the C 1 and C 3v sites, respectively.To the best of our knowledge, there are no reported R values for the Y 2 O 2 SO 4 host, but the R value for Y 2 O 2 S was found to be approximately 6.50, which is consistent with our calculated value. 58ext, to explore the Y 2 O 2 SO 4 phase transformation in situ, the samples were annealed in an H 2 /N 2 (5:95) gas in the TGA/DSC apparatus with a ramp rate of 5 °C/min (Figure 3a).A small (2%) reduction of mass is measured below 400 °C and is associated with the physisorbed/chemisorbed species (water and toluene) used to wash the samples.The diffraction pattern for an equivalent sample (Figure 3b) exhibits no change in structure at 400 °C confirming the loss of surface species during low-temperature treatments.Further annealing of the samples shows minimal weight loss and heat flux until 600 °C.At this temperature, a small endothermic peak is observed followed by a rapid loss in mass from 600 to 800 °C.
The first reduction is due to the extraction of oxygen ions from the sulfate group resulting in the conversion of Y 2 O 2 SO 4 to Y 2 O 2 S.This reaction can be written as This agrees well with a previous experiment in which Y 2 O 2 SO 4 samples were reduced to Y 2 O 2 S when calcined at 600 °C under H 2 /N 2 flow (with a dwell time of 2 h), as shown in Figure 3b.The complete conversion of Y 2 O 2 SO 4 to Y 2 O 2 S is expected to result in a 20.9% change in mass.However, the total mass loss between 600 and 800 °C was experimentally observed to be approximately 24.2%, suggesting a secondary transformation is occurring, i.e., the decomposition of Y 2 O 2 SO 4 to Y 2 O 3 .This transformation would result in a 26.9% theoretical mass loss, indicating that in our TGA/DSC experiment, the rate of decomposition is much slower than the rate of reduction at higher temperatures (600−800 °C).This agrees well with the XRD data for 800 °C where mainly Y 2 O 3 peaks, with an impurity Y 2 O 2 S crystal phase, were observed (Figure 3b).A reference reaction in N 2 shows no mass or phase change up to 600 °C confirming the H 2 -induced reduction (Figure 3b).
Next, the samples were doped with Tb 3+ (5 mol %) and Tb 3+ /Ce 3+ (5 mol %/5 mol %) to understand the role of the different hosts on the energy transfer and luminescence.Figure 4a shows the photoluminescence emission and excitation (PLE) and PL spectra of Y 2 O 2 SO 4 (5 mol % Tb 3+ ) and Y 2 O 2 SO 4 (5 mol % Tb 3+ , 5 mol % Ce 3+ ).The excitation spectra are normalized to the RE-O/RE-S CTB in order to eliminate the intensity variations due to sample loading.The excitation peaks of the codoped host at 258 and 285 nm are ascribed to electric dipole-allowed f−d transitions of the Ce 3+ and Tb 3+ ions. 16,59The excitation peak above 300 nm corresponds to f−d transitions of the Ce 3+ and f−f transitions of Tb 3+ . 60,61Furthermore, the emission spectra were collected at 327 nm excitation.In both samples, the characteristic Tb 3+ emission bands at 485 ( 5 D 4 → 7 F 6 ), 542 ( 5 D 4 → 7 F 5 ), 585 ( 5 D 4 → 7 F 4 ), and 620 nm ( 5 D 4 → 7 F 3 ) were observed.However, the codoped sample has a significantly higher intensity than the singly doped sample.This is indicative of the energy transfer from Ce 3+ to Tb 3+ .To verify this, lowtemperature (77 K) lifetime measurements were performed (Figure 4b).The shape of the white line region (at 2 ms), i.e., short increase before decay onset, is a signature of energy transfer between two activator ions or between the sensitizer and activator. 62As can be seen from the inset of Figure 4b, the Tb 3+ doped samples show an immediate, rapid decay commonly seen with single absorber/emitter systems.The low-temperature lifetime measurements indicate that the RE 3+ dopant is demonstrating high efficiencies based on comparable Tb systems. 19,48The comparative assessment of lifetime values within codoped and singly doped samples revealed a discernible increase in the former.This observed enhancement, as demonstrated in prior investigations concerning Tb 3+/ Ce 3+ doped systems, 17,63 was elucidated through the conceivable occurrence of energy transfer mechanisms between the two distinct dopants due to the inherent symmetry-driven energy transfer dynamics.Another plausible explanation for the energy transfer can be the change in the phonon dispersion of the lattice with the replacement of the anion with SO 4 2− which was explored in the previous Tb 3+ doped oxyhalide systems. 64hen Tb 3+ and Ce 3+ ions are in proximity, their energy states can be coupled with the phonon modes of the crystal lattice.This coupling enables the transfer of energy between dopants, modifying their excited state lifetimes.
Additionally, the lifetime measurements for the other hosts (Y 2 O 2 S and Y 2 O 3 ) were recorded as shown in Figures S3a−d and S4a−d, and both hosts showed similar behavior for the  2).Compared with Y 2 O 2 SO 4 , Ce 3+ /Tb 3+ -doped Y 2 O 2 S exhibited lifetime values lower than those of the Tb 3+ -doped sample, indicating no energy transfer between the dopants.In fact, luminescence is quenched when Ce 3+ is codoped into Y 2 O 2 S due to the trigonal crystal dopant sites (C 3v symmetry) modifying the dopant crystal field splitting and limited overlap of the 5d level with the Tb 3+5 D 4 → 7 F 5 energy levels. 65As a result, a reverse energy transfer from the 5 D 4 energy level of Tb 3+ to the 5d energy level of Ce 3+ can occur.−67 At 285 nm excitation, a lifetime value for the codoped Y 2 O 3 sample could not be calculated due to a weak emission intensity resulting from significant quenching due to parasitic surface sites. 68urthermore, the 285 nm excitation of Y 2 O 3 (5 mol % Tb 3+ ) has a shorter lifetime that is due to its cubic phase crystal structure with Tb 3+ ions occupying the high symmetry S 6 site. 69

CONCLUSIONS
In summary, Y 2 O 2 SO 4 :RE 3+ (Eu 3+ , Tb 3+ , Tb 3+ /Ce 3+ ) has been successfully prepared via a low-temperature two-step hydrothermal/combustion method.The as-synthesized samples were further reduced to Y 2 O 2 S under an H 2 /N 2 flow at 600 °C.XRD, FTIR, and PL measurements were used to extract the structural properties, the nature of bonding in the crystal lattices, and relationships between structural and photoluminescent properties, respectively.Rietveld refinement of the XRD data confirmed the crystallographic and structural parameters of all three hosts (Y 2 O 2 SO 4 , Y 2 O 2 S, and Y 2 O 3 ).The chemical transformations under different environments have been examined by TGA/DSC measurements, and the phases obtained were confirmed by XRD.Ultimately, Tb 3+ and Ce 3+ doping was employed to study energy transfer mechanisms in three hosts with different crystal structures.PL spectra and lifetime measurements show that the Y 2 O 2 SO 4 host maximizes the energy transfer from Ce 3+ to Tb 3+ and enhances the green emission of Tb 3+ ions due to the structure altering both the local symmetry/environment, phonon dispersion around the RE ions and the relative position of the Ce 3+ CTB with respect to the Tb 3+ energy levels.These results indicate that photoluminescent properties of RE-doped oxides can be tailored via anionic substitution, which can allow for the design of novel phosphor materials with enhanced properties for the application of w-LEDs, solid-state lasers, and noncontact thermometers.

Figure 2 .
Figure 2. (a) XRD patterns of 5 mol % Eu3+  -doped hydrothermally synthesized precursor/sulfur at 500 °C with air (S1), 5 mol % Eu 3+ doped hydrothermally synthesized precursor/sulfur at 500 °C with air in a sealed quartz tube (S2) and the reduction of as-synthesized S@ at 600 °C with 5% H 2 /N 2 flow (S3).(b) FTIR spectra of samples S1, S2, and S3, which shows the IR active vibrations for the different hosts.(c) Excitation spectra of S1, S2, and S3 at 611 nm (only for S1) and 617 nm emission and emission spectra of S1, S2, and S3 at 395 nm excitation.(d) Relations of the wavelength of the electric dipole transition ( 5 D 0 → 7 F 2 ) at the peak maximum and the asymmetric ratios (R) as a function of the S/O molar ratios.
After that, a milky solution was transferred to a Teflon liner autoclave and was heated up to 160 °C overnight.Then, the sample was washed with deionized water several times and dried at 100 °C overnight.This hydrothermally synthesized intermediate and sulfur were then reacted in air without sealing at 500 °C for 4 h with a 10 °C/min ramp rate in order to obtain crystalline Y 2 O 3 :Tb 3+ , Ce 3+ .Y 2 O 2 SO 4 :RE 3+ was obtained by mixing the hydrothermally synthesized intermediate (Y(OH) 3

Table 1 .
56ystallographic Data and Structure Parameters from Rietveld Refinement (All Samples Doped with 5 mol % Eu 3+ ) dependent.56LargerR values are indicative of lower local symmetry and better luminescence.The highest R ratio (14.7) was observed in the Y 2 O 3 .Usually, Eu 3+ ions occupy either C 2 or S 6 sites in the cubic phase Y 2 O 3 host; the latter one has more symmetric coordination than the former sites.Here, the largest asymmetry ratio designates Eu 3+ ions to occupy mainly C 2 sites in the Y 2 O 3 , which agrees well with previously reported values.

AUTHOR INFORMATION Corresponding Author
Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States Kelly E. Cohen − Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States Figure 4. Photoluminescent properties of Y 2 O 2 SO 4 (5 mol % Tb 3+ ) and Y 2 O 2 SO 4 (5 mol % Tb 3+ , 5 mol % Ce 3+ ).(a) PL excitation spectra at an emission wavelength of 542 nm and PL emission spectra excited at 327 nm.(b) Logarithmic lifetime plots of Y 2 O 2 SO 4 (5 mol % Tb 3+ ) and Y 2 O 2 SO 4 (5 mol % Tb 3+ , 5 mol % Ce 3+) with an excitation wavelength of 327 nm and an emission wavelength of 542 nm at 77 K.