Hydrothermal Synthesis, Crystal Structure, and Spectroscopic Properties of Pure and Eu3+-Doped NaY[SO4]2 ∙ H2O and Its Anhydrate NaY[SO4]2

The water-soluble colorless compound NaY[SO4]2 ∙ H2O was synthesized with wet methods in a Teflon autoclave by adding a mixture of Na2[SO4] and Y2[SO4]3 ∙ 8 H2O to a small amount of water and heating it up to 190 °C. By slow cooling, single crystals could be obtained and the trigonal crystal structure of NaY[SO4]2 ∙ H2O was refined based on X-ray diffraction data in space group P3221 (a = 682.24(5) pm, c = 1270.65(9) pm, Z = 3). After its thermal decomposition starting at 180 °C, the anhydrate NaY[SO4]2 can be obtained with a monoclinic crystal structure refined from powder X-ray diffraction data in space group P21/m (a = 467.697(5) pm, b = 686.380(6) pm, c = 956.597(9) pm, β = 96.8079(5), Z = 2). Both compounds display unique Y3+-cation sites with eightfold oxygen coordination (d(Y–Os = 220–277 pm)) from tetrahedral [SO4]2− anions (d(S–O = 141–151 pm)) and a ninth oxygen ligand from an H2O molecule (d(Y–Ow = 238 pm) in the hydrate case. In both compounds, the Na+ cations are atoms (d(Na–Os = 224–290 pm) from six independent [SO4]2− tetrahedra each. Thermogravimetry and temperature-dependent PXRD experiments were performed as well as IR and Raman spectroscopic studies. Eu3+-doped samples were investigated for their photoluminescence properties in both cases. The quantum yield of the red luminescence for the anhydrate NaY[SO4]2:Eu3+ was found to be almost 20 times higher than the one of the hydrate NaY[SO4]2 ∙ H2O:Eu3+. The anhydrate NaY[SO4]2:Eu3+ exhibits a decay time of about τ1/e = 2.3 µm almost independent of the temperature between 100 and 500 K, while the CIE1931 color coordinates at x = 0.65 and y = 0.35 are very temperature-consistent too. Due to these findings, the anhydrate is suitable as a red emitter in lighting for emissive displays.

The anhydrous oxosulfate NaY[SO4]2 can be obtained by heating NaY[SO4]2 air at a temperature of 180 °C or higher (Equation (3)). The powder, which was the crystal structure refinement, was drained at 550 °C. For the luminescence m ments, a Eu 3+

X-ray Experiments and Crystal-Structure Solution
For single-crystal X-ray diffraction experiments, a suitable crystal was select a light microscope and fixed inside of a glass capillary with an outer diameter o and a length of about 15 mm. The crystal was measured with a κ-CCD four-cir diffractometer (Bruker Nonius, Karlsruhe, Germany) with Mo-Kα radiation (λ = 7 at 293 K (room temperature). Crystal-structure solution and refinement for Na H2O (CSD-2016596) in the trigonal space group P3221 were carried out with the package SHELX-97 [36,37] by Sheldrick, and the program HABITUS by Bärnin and Herrendorf was applied [38] for a numerical absorption correction.
For X-ray powder diffraction (PXRD), part of the sample was fixed on a diffractometer (Stoe & Cie, Darmstadt, Germany) and measured with Cu-Kα rad = 154.06 pm) in transmission setting. The monohydrate was measured from 2ϴ for checking phase purity and the anhydrate NaY[SO4]2 (CSD-2072719) was m from 2ϴ = 8-110° for solving its crystal structure in the NaEr[SO4]2-type arrangem with the program FULLPROF [39,40]. The measured powder X-ray diffraction p NaY[SO4]2 • H2O can be seen in Figure 2 (top) and the measured PXRD pattern with the difference plot of the Rietveld refinement for NaY[SO4]2 is shown in (bottom). The anhydrous oxosulfate NaY[SO 4 ] 2 can be obtained by heating NaY[SO 4 ] 2 · H 2 O in air at a temperature of 180 • C or higher (Equation (3)). The powder, which was used for the crystal structure refinement, was drained at 550 • C. For the luminescence measurements, a Eu 3+

X-ray Experiments and Crystal-Structure Solution
For single-crystal X-ray diffraction experiments, a suitable crystal was selected under a light microscope and fixed inside of a glass capillary with an outer diameter of 0.1 mm and a length of about 15 mm. The crystal was measured with a κ-CCD four-circle X-ray diffractometer (Bruker Nonius, Karlsruhe, Germany) with Mo-Kα radiation (λ = 71.07 pm) at 293 K (room temperature). Crystal-structure solution and refinement for NaY[SO 4 ] 2 · H 2 O (CSD-2016596) in the trigonal space group P3 2 21 were carried out with the program package SHELX-97 [36,37] by Sheldrick, and the program HABITUS by Bärninghausen and Herrendorf was applied [38] for a numerical absorption correction.
For X-ray powder diffraction (PXRD), part of the sample was fixed on a STADI-P diffractometer (Stoe & Cie, Darmstadt, Germany) and measured with Cu-Kα radiation (λ = 154.06 pm) in transmission setting. The monohydrate was measured from 2θ = 10-90 • for checking phase purity and the anhydrate NaY[SO 4 ] 2 (CSD-2072719) was measured from 2θ = 8-110 • for solving its crystal structure in the NaEr[SO 4 ] 2 -type arrangement [31] with the program FULLPROF [39,40]. The measured powder X-ray diffraction pattern of NaY[SO 4 ] 2 · H 2 O can be seen in Figure 2 (top) and the measured PXRD pattern together with the difference plot of the Rietveld refinement for NaY[SO 4 ] 2 is shown in Figure 2 (bottom). Temperature-depending powder X-ray diffraction data were measured in t val 2ϴ = 10-90° with a RIGAKU SmartLab diffractometer (Neu-Isenburg, German Cu-Kα radiation (λ = 154.06 pm) in reflection setting from 25 up to 900 °C.
While all the atomic displacement parameters of NaY[SO4]2 • H2O could be anisotropically based on single-crystal X-ray diffraction data, the atomic displace rameters of NaY[SO4]2 were only treated isotropically with Rietveld refinement b PXRD data. Temperature-depending powder X-ray diffraction data were measured in the interval 2θ = 10-90 • with a RIGAKU SmartLab diffractometer (Neu-Isenburg, Germany) using Cu-Kα radiation (λ = 154.06 pm) in reflection setting from 25 up to 900 • C.

Thermal Analysis
While all the atomic displacement parameters of NaY[SO 4 ] 2 · H 2 O could be refined anisotropically based on single-crystal X-ray diffraction data, the atomic displacement parameters of NaY[SO 4 ] 2 were only treated isotropically with Rietveld refinement based on PXRD data.

Thermal Analysis
Thermal analysis (thermogravimetry) was performed with about 36 mg of a NaY[SO 4 ] 2 · H 2 O samples with a Netzsch device of the type STA-449C (Selb, Germany) in a corundum crucible under argon atmosphere. The sample was heated with 5 K/min from 25 to 1400 • C.

Luminescence Spectroscopy
Excitation and emission spectra were collected using a fluorescence spectrometer FLS920 (Edinburgh Instruments, Livingston, UK) equipped with a 450 W ozone-free xenon discharge lamp (Osram, München, Germany) and a cryostat "MicrostatN" from Oxford Instruments (Abingdon, UK) as the sample chamber. Additionally, a mirror optic for powder samples was applied. For detection, an R2658P single-photon-counting photomultiplier tube (Hamamatsu, Hamamatsu, Japan) was used. All photoluminescence spectra were recorded with a spectral resolution of 0.5 nm and a dwell time of 0.5 s in 0.5 nm steps.
The photoluminescence decay times were measured on an FLS920 spectrometer (Edinburgh Instruments, Livingston, UK). A Xe µ-flash lamp µF920 was used as an excitation source. For detection, an R2658P single-photon-counting photomultiplier tube (Hamamatsu Photonics, Hamamatsu, Japan) found application.
For the reflection spectra, the investigated samples were placed into an integrating sphere, and FLS920 spectrometer (Edinburgh Instruments, Livingston, UK) equipped with a 450 W Xe lamp, and a cooled (−20 • C) single-photon-counting photomultiplier (Hamamatsu R928) was used. Ba [SO 4 ] was applied as the reflectance standard. The excitation and emission bandwidths were 10.00 and 0.06 nm, respectively.
Step width was 0.5 nm and integration time 0.5 s.
Quantum yields were determined according to the method published by Kawamura et al. [41] upon excitation at 395 nm using a 7 nm excitation and 0.5 nm emission slit. The scan steps were 0.5 nm, while the respective emission intensity from 370 to 750 nm was recorded.
The CIE1931 color coordinates and luminous efficacy (LE) values were calculated from the temperature-dependent emission spectra of NaY[SO 4 ] 2 :Eu 3+ using the Color Calculator 6.75 software from Osram (Osram, München, Germany) [42].
The LE value (unit: lm/W) is a parameter describing, how bright the radiation is perceived by an average human observer at a photopic illumination situation. It scales with the photopic human eye sensitivity curve V(λ) and can be calculated from the normalized emission spectrum I(λ) of the sample as follows [43]:

IR and Raman Spectra
Infrared spectra for powder samples of NaY[SO 4 ] 2 · H 2 O and NaY[SO 4 ] 2 was measured from 700 to 4000 cm −1 with a NICOLET iS5 device from Thermo Scientific (Karlsruhe, Germany). Raman spectroscopy was performed with a DXR SmartRaman spectrometer from Thermo Scientific (Karlsruhe, Germany) with a red laser (λ = 780 nm) and a laser power of 10 mW from 200 to 1800 cm −1 .     Figure 3.    [20][21][22][23], it further coordinates the alkali-metal cation. The crystal structure of the anhydrous potassium rare-earth metal oxosulfates are described triclinically in space group P1 for RE = Pr [47] and Nd [48], but monoclinically in space group P2 1 /c for RE = Nd [49] and Er [50]. In the triclinic structure, the coordination number of RE 3+ is eight, while in the monoclinic one, it surprisingly increases to nine. Two oxosulfate anions coordinate with two oxygen atoms each in the monoclinic KRE[SO 4 ] 2 representatives, while in the triclinic cases only one [SO 4 ] 2group has two contacts to the rare-earth metal cations. The coordination environments of those in the two title compounds are compared to the other alkali-metal rare-earth metal oxosulfates in Figure 5.  In Y2[SO4]3 • 8 H2O [45], the unique Y 3+ cations are also surrounded by eight oxygen atoms (four from water molecules and four more from oxosulfate anions) with distances between 230 and 247 pm, while in the anhydrous oxosulfate Y2[SO4]3, Y 3+ is surrounded octahedrally by only six oxygen atoms from oxosulfate groups with distances between 220 and 224 pm [46]. While Y 3+ is coordinated by just one oxygen atom per [SO4] 2- [20][21][22][23], it further coordinates the alkali-metal cation. The crystal structure of the anhydrous potassium rare-earth metal oxosulfates are described triclinically in space group P1 for RE = Pr [47] and Nd [48], but monoclinically in space group P21/c for RE = Nd [49] and Er [50]. In the triclinic structure, the coordination number of RE 3+ is eight, while in the monoclinic one, it surprisingly increases to nine. Two oxosulfate anions coordinate with two oxygen atoms each in the monoclinic KRE[SO4]2 representatives, while in the triclinic cases only one [SO4] 2-group has two contacts to the rare-earth metal cations. The coordination environments of those in the two title compounds are compared to the other alkali-metal rare-earth metal oxosulfates in Figure 5.

Results and Discussion
The sodium cations in both title compounds are surrounded by eight oxygen atoms from six different oxosulfate units as a bicapped octahedron. While Na + in NaY[SO4]2 • H2O is only connected with [SO4] 2− anions and no water molecules, in the related potas-  The sodium cations in both title compounds are surrounded by eight oxygen atoms from six different oxosulfate units as a bicapped octahedron. While Na + in NaY[SO 4 ] 2 · H 2 O is only connected with [SO 4 ] 2− anions and no water molecules, in the related potassium compound the K + cation has contact with six of them and one water molecule [20][21][22][23]. The anhydrous potassium rare-earth metal oxosulfates show a coordination sphere around the alkali-metal cation erected by ten oxygen atoms from six oxosulfate anions in case of the triclinic examples [47,48] and seven terminal [SO 4 ] 2units in the monoclinic cases [49,50]. In the orthorhombic salt Na 2 [SO 4 ], the sodium cations show six oxygen atoms from five oxosulfate groups as next neighbors [51], while in its decahydrate Na 2 [SO 4 ] · 10 H 2 O, Na + is only surrounded by six water molecules octahedrally [52] (Figure 6).  While NaY[SO4]2 • H2O exhibits only one singular crystallographic [SO4] 2− anion, its anhydrate has two different ones of them ( Figure 7). All oxygen atoms in NaY[SO4]2 • H2O are surrounded approximately in a plane triangular fashion by Y 3+ , Na +, and S 6+ , while in NaY[SO4]2 O2 and O6 differ from this scheme since O2 is coordinated by one S 6+ and two Y 3+ and O6 by one S 6+ and two Na + cations. Even O5w has one Y 3+ and two H + cations, three neighbors. The triangular environments of the oxygen atoms in NaY[SO4]2 • H2O and NaY[SO4]2 can be seen in Figure 8. Selected interatomic distances (d/pm) are summarized in Table 3. While NaY[SO 4 ] 2 · H 2 O exhibits only one singular crystallographic [SO 4 ] 2− anion, its anhydrate has two different ones of them ( Figure 7). All oxygen atoms in NaY[SO 4 ] 2 · H 2 O are surrounded approximately in a plane triangular fashion by Y 3+ , Na +, and S 6+ , while in NaY[SO 4 ] 2 O2 and O6 differ from this scheme since O2 is coordinated by one S 6+ and two Y 3+ and O6 by one S 6+ and two Na + cations. Even O5w has one Y 3+ and two H + cations, three neighbors. The triangular environments of the oxygen atoms in NaY[SO 4 ] 2 · H 2 O and NaY[SO 4 ] 2 can be seen in Figure 8. Selected interatomic distances (d/pm) are summarized in Table 3.         [53] is v ij = exp[(R ij -d ij )/b] with the valence v ij , the universal constant b = 0.37 Å, the bond-valence parameter R ij, and the Ångström distance of the considered atoms d ij between the atoms i and j. The sum ∑(v ij ) represents the charge of the regarded ion.  Table 5.

Thermal Analysis
A thermogravimetrical curve for the decomposition of NaY[SO 4 ] 2 · H 2 O between 25 and 1400 • C is depicted in Figure 9.
We became aware of a competing structure refinement for trigonal NaY[SO (a = 681.91(3) pm, c = 1270.35 (11) pm, c/a = 1.863) in space group P3121 that was al the progress of publication [54], simultaneous to our activities writing this arti lower CSD deposition number (ours for NaY[SO4]2 • H2O in space group P3221: versus the Chinese competitor one for NaY[SO4]2 • H2O in space group P3121: 2 should grant us a priority, despite the almost identical results in both papers from 2021.

Thermal Analysis
A thermogravimetrical curve for the decomposition of NaY[SO4]2 • H2O bet and 1400 °C is depicted in Figure 9.    Figure S3). The TG curve (Figure 9) appears to be similar to that of NaRE[SO 4 ] 2 · H 2 O with RE = La, Ce, Nd, and Sm, which have been measured in 1994 by Kolcu and Zümreoǧlu-Karan [18]. The dehydration temperature of the lanthanum compound is 297 • C and gets lower with decreasing RE 3+ -cation radius along with the lanthanoid contraction [61]. With a dehydration temperature of 265 • C for the samarium compound, this trend is further confirmed with our yttrium analog at 180 • C.
Additional to the thermogravimetry, temperature-dependent X-ray diffraction experiments were performed ( Figure 10). and Sm, which have been measured in 1994 by Kolcu and Zümreoǧlu-Karan [18]. The dehydration temperature of the lanthanum compound is 297 °C and gets lower with decreasing RE 3+ -cation radius along with the lanthanoid contraction [61]. With a dehydration temperature of 265 °C for the samarium compound, this trend is further confirmed with our yttrium analog at 180 °C.
Additional to the thermogravimetry, temperature-dependent X-ray diffraction experiments were performed ( Figure 10). 10

Luminescence-Spectroscopic Properties
Eu 3+ -doped samples of NaY[SO4]2 • H2O and NaY[SO4]2 under UV irradiation (λ = 254 nm) can be seen in Figure 11. Both compounds display a reflection spectrum, which is in line with plain white powders of good optical quality and high crystallinity, due to the lack of greying or defect bands. The absorption edge of the anhydrous compound at about 270 nm is assigned to the LMCT (ligand-to-metal charge-transfer) absorption band of Eu 3+ , which is a typical Both compounds display a reflection spectrum, which is in line with plain white powders of good optical quality and high crystallinity, due to the lack of greying or defect bands. The absorption edge of the anhydrous compound at about 270 nm is assigned to the LMCT (ligand-to-metal charge-transfer) absorption band of Eu 3+ , which is a typical energetic position of the LMCT process of Eu 3+ in an oxidic environment [62]. The reflectance values at longer wavelengths were close to unity, pointing to a high optical quality of the prepared materials. In both reflection spectra (Figure 12), the typical Eu 3+ absorption lines originating from the 7 F 0 → 5 L 6 and 7 F 0 → 5 D 2 transitions could be observed in the ranges of 395-397 nm and 450-470 nm, respectively [63]. Both compounds display a reflection spectrum, which is in lin powders of good optical quality and high crystallinity, due to the lack bands. The absorption edge of the anhydrous compound at about 27 the LMCT (ligand-to-metal charge-transfer) absorption band of Eu 3+ energetic position of the LMCT process of Eu 3+ in an oxidic environm tance values at longer wavelengths were close to unity, pointing to a of the prepared materials. In both reflection spectra (Figure 12), the typ lines originating from the 7 F0 → 5 L6 and 7 F0 → 5 D2 transitions could ranges of 395-397 nm and 450-470 nm, respectively [63].   [63,64] and the position of the LMCT of Eu 3+ in the anhydr LMCT band was located at 270 nm and was in good agreement with t   [63,64] and the position of the LMCT of Eu 3+ in the anhydrous compound. The LMCT band was located at 270 nm and was in good agreement with the position derived from the reflection spectrum. The excitation spectra of both compounds are plotted in Figure 13.
Noteworthy was the temperature-dependent excitation spectra of NaY[SO 4 ] 2 · H 2 O:Eu 3+ , since a closer look at the UV-A range revealed a distinct change of the pattern of 7 F 0 → 5 L 6 (390-405 nm) and 7 F 0 → 5 L 8 + 5 G J + 5 L 9 + 5 L 10 (J = 2-6) (373-387 nm) transitions [65]. The thermal population of the 7 F 1 level could explain some changes in the excitation line pattern. However, the shift and broadening of the most intense line of the 7 F 0 → 5 L 6 multiplet at 394 nm from 400 K onwards pointed to a phase transition. This finding could be explained by the loss of water and the transformation of NaY[SO 4 ] 2 · H 2 O:Eu 3+ to NaY[SO 4 ] 2 :Eu 3+ in good accordance with the results from thermal gravimetry (Figure 9). Temperature-dependent emission spectra of NaY[SO 4 ] 2 · H 2 O:Eu 3+ and NaY[SO 4 ] 2 :Eu 3+ are shown in Figure 14. from the reflection spectrum. The excitation spectra of both compounds are plotted in Figure 13. Noteworthy was the temperature-dependent excitation spectra of NaY[SO4]2 • H2O:Eu 3+ , since a closer look at the UV-A range revealed a distinct change of the pattern of 7 F0 → 5 L6 (390-405 nm) and 7 F0 → 5 L8 + 5 GJ + 5 L9 + 5 L10 (J = 2-6) (373-387 nm) transitions [65]. The thermal population of the 7 F1 level could explain some changes in the excitation line pattern. However, the shift and broadening of the most intense line of the 7 F0 → 5 L6 multiplet at 394 nm from 400 K onwards pointed to a phase transition. This finding could be explained by the loss of water and the transformation of NaY[SO4]2 • H2O:Eu 3+ to NaY[SO4]2:Eu 3+ in good accordance with the results from thermal gravimetry (Figure 9). Temperature-dependent emission spectra of NaY[SO4]2 • H2O:Eu 3+ and NaY[SO4]2:Eu 3+ are shown in Figure 14. The emission spectra of Eu 3+ -comprising materials consisted of the orange allowed magnetic-dipole (MD) transition 5 D 0 → 7 F 1 , the red parity-forbidden electric-dipole (ED) transition 5 D 0 → 7 F 2 , and further line multiplets in the deep red spectral range around 650 and 695 nm due to the ED transitions 5 D 0 → 7 F 3 and 5 D 0 → 7 F 4 . For light sources and emissive displays, the emission spectrum should consist mainly of emission lines resulting from the 5 D 0 → 7 F 2 transitions [66,67]. This means that the Eu 3+ cation has to occupy a crystallographic site without inversion symmetry (see Figure 4 for symmetry examination). This also induces the deep red emission lines. Fortunately, the 5 D 0 → 7 F 2 transition is hypersensitive and small deviations of the inversion symmetry strongly enhance the probability of the 5 D 0 → 7 F 2 transitions. The intensity of the strongly forbidden transition 5 D 0 → 7 F 0 is known to correlate with the linear terms of the crystal-field parameter and polarizability of the Eu 3+ cation [67].
However, the emission spectrum of NaY[SO 4 ] 2 · H 2 O:Eu 3+ upon 395 nm excitation revealed the typical emission line pattern between 580 and 720 nm due to the 5 D 0 → 7 F J (J = 0-4) transitions of Eu 3+ [62,63,68]. Unfortunately, the signal-to-noise ratio is rather low, which points to a low quantum yield. Indeed, the determination of the quantum efficiency according to Kawamura [41] yielded a value of solely about 1%. Such a low quantum yield can be explained by the presence of crystal water since the high phonon frequency of the O-H vibration of water quenches efficiently the Eu 3+ luminescence [69].  The emission spectra of Eu 3+ -comprising materials consisted of the orange allowed magnetic-dipole (MD) transition 5 D0 → 7 F1, the red parity-forbidden electric-dipole (ED) transition 5 D0 → 7 F2, and further line multiplets in the deep red spectral range around 650 and 695 nm due to the ED transitions 5 D0 → 7 F3 and 5 D0 → 7 F4. For light sources and emissive displays, the emission spectrum should consist mainly of emission lines resulting from the 5 D0 → 7 F2 transitions [66,67]. This means that the Eu 3+ cation has to occupy a crystallographic site without inversion symmetry (see Figure 4 for symmetry examination). This also induces the deep red emission lines. Fortunately, the 5 D0 → 7 F2 transition is hypersensitive and small deviations of the inversion symmetry strongly enhance the probability of the 5 D0 → 7 F2 transitions. The intensity of the strongly forbidden transition 5 D0 → 7 F0 is known to correlate with the linear terms of the crystal-field parameter and polarizability of the Eu 3+ cation [67].
However, the emission spectrum of NaY[SO4]2 • H2O:Eu 3+ upon 395 nm excitation revealed the typical emission line pattern between 580 and 720 nm due to the 5 D0 → 7 FJ (J = 0-4) transitions of Eu 3+ [62,63,68]. Unfortunately, the signal-to-noise ratio is rather low, which points to a low quantum yield. Indeed, the determination of the quantum efficiency according to Kawamura [41] yielded a value of solely about 1%. Such a low quantum yield can be explained by the presence of crystal water since the high phonon frequency of the O-H vibration of water quenches efficiently the Eu 3+ luminescence [69]. As already observed for the excitation spectra, the temperature-dependent emission spectra of NaY[SO 4 ] 2 · H 2 O:Eu 3+ showed a distinct change once the temperature exceeded 400 K, resulting in the increase of intensity and the width of the 5 D 0 → F 1 , 5 D 0 → 7 F 2 , and 5 D 0 → 7 F 4 transitions, as well as the appearance of the 5 D 0 → 7 F 0 transition, which was absent at room temperature. This change again points to a phase transition, i.e., the transformation of NaY[SO 4 ] 2 · H 2 O:Eu 3+ to NaY[SO 4 ] 2 :Eu 3+ , which goes along with an increase of the crystal-field strength causing a larger energetic spread of the Stark components of the above mentioned 5 D 0 → 7 F J , transitions. This finding was in good agreement with the decline of the coordination number from 9 to 8 and a shorter average Y-O distance. However, even though the emission spectra of the anhydrous sample obtained after the phase transition resembled that of the as-prepared anhydrous sample, the emission spectra were not completely the same. We assumed that after the phase transition a higher defect density remained, which resulted in line-broadening and a lower signal-to-noise ratio since, without further high-temperature treatment, defects caused by the water removal cannot be healed. In contrast, the as-prepared samples of anhydrous NaY[SO 4 ] 2 :Eu 3+ showed a much higher quantum yield. This value was determined to be almost around 20%, which also explained the much better signal-to-noise ratio of the respective emission spectra as a function of temperature (Figure 14, bottom).
The CIE1931 color coordinates of NaY[SO 4 ] 2 :Eu 3+ are at x = 0.65 and y = 0.35, while the temperature impact is rather low, branding the substance as a stable color-consistent material for application in displays or fluorescent light sources [1]. However, the magnification of the color space in Figure 15 demonstrates that the color point shifts slightly to the orange range, which can be caused by the reduction of the asymmetry ratio 5 D 0 → 7 F 2 / 5 D 0 → 7 F 1 [63] or by the reduction of the covalency related to the 5 D 0 → 7 F 4 / 5 D 0 → 7 F J ratio [3]. However, both effects are in line with a thermal expansion of the crystals and the Eu 3+ site causes a decrease of the covalent interaction between Eu 3+ and oxygen and an increase of the local symmetry. above mentioned 5 D0 → 7 FJ, transitions. This finding was in good agreement with the decline of the coordination number from 9 to 8 and a shorter average Y-O distance. However, even though the emission spectra of the anhydrous sample obtained after the phase transition resembled that of the as-prepared anhydrous sample, the emission spectra were not completely the same. We assumed that after the phase transition a higher defect density remained, which resulted in line-broadening and a lower signal-to-noise ratio since, without further high-temperature treatment, defects caused by the water removal cannot be healed. In contrast, the as-prepared samples of anhydrous NaY[SO4]2:Eu 3+ showed a much higher quantum yield. This value was determined to be almost around 20%, which also explained the much better signal-to-noise ratio of the respective emission spectra as a function of temperature (Figure 14, bottom).
The CIE1931 color coordinates of NaY[SO4]2:Eu 3+ are at x = 0.65 and y = 0.35, while the temperature impact is rather low, branding the substance as a stable color-consistent material for application in displays or fluorescent light sources [1]. However, the magnification of the color space in Figure 15 demonstrates that the color point shifts slightly to the orange range, which can be caused by the reduction of the asymmetry ratio 5 D0 → 7 F2 / 5 D0 → 7 F1 [63] or by the reduction of the covalency related to the 5 D0 → 7 F4/ 5 D0 → 7 FJ ratio [3]. However, both effects are in line with a thermal expansion of the crystals and the Eu 3+ site causes a decrease of the covalent interaction between Eu 3+ and oxygen and an increase of the local symmetry. Noteworthy were the intensities of the temperature-dependent emission spectra of NaY[SO4]2 • H2O:Eu 3+ as depicted in Figure 16. While the intensity decreased between 100 and 300 K due to typical thermal quenching, it increased again between 300 and 500 K. This effect was caused by the phase transition towards the formation of the more efficiently luminescent NaY[SO4]2:Eu 3+ upon increasing the temperature. Noteworthy were the intensities of the temperature-dependent emission spectra of NaY[SO 4 ] 2 · H 2 O:Eu 3+ as depicted in Figure 16. While the intensity decreased between 100 and 300 K due to typical thermal quenching, it increased again between 300 and 500 K. This effect was caused by the phase transition towards the formation of the more efficiently luminescent NaY[SO 4 ] 2 :Eu 3+ upon increasing the temperature. In contrast, the temperature-dependent emission spectra of NaY[SO4]2:Eu 3+ itself show a typical decrease of the intensity or quantum yield of Eu 3+ phosphors with increasing temperature [63].
Finally, we investigated the time-dependent luminescence (Figure 17) of the 5 D0 → 7 F2 transition of Eu 3+ at 617 nm upon 395 nm excitation of NaY[SO4]2 • H2O:Eu 3+ and NaY[SO4]2:Eu 3+ . As discussed before, NaY[SO4]2 • H2O:Eu 3+ shows a peculiar behavior due to the phase transition between 400 and 500 K, which means that the decay time increases In contrast, the temperature-dependent emission spectra of NaY[SO 4 ] 2 :Eu 3+ itself show a typical decrease of the intensity or quantum yield of Eu 3+ phosphors with increasing temperature [63].
Finally, we investigated the time-dependent luminescence ( Figure 17) of the 5 3+ shows a peculiar behavior due to the phase transition between 400 and 500 K, which means that the decay time increases from 550 µs at 100 K to about 930 µs at 500 K. At the same time, the decay curves become bi-exponential, which points to the formation of a novel phase with a prolonged decay time and enhanced internal quantum efficiency. The decay curves of NaY[SO 4 ] 2 :Eu 3+ between 100 and 500 K were almost perfectly mono-exponential over three orders of magnitude, while the derived decay times remained rather constant, as proven by the just slight decline from 2.35 ms to 2.20 ms. This finding meant that the internal quantum yield stayed quite stable, and thus, thermal quenching of the Eu 3+ photoluminescence is a minor issue.   Figure 18 and the values are given in Table 6 compared to the literature data for Y 2 [SO 4 ] 3 [69] and Na 2 [SO 4 ] (thenardite) [70]. The vibration at about 2300 cm -1 represents CO 2

Conclusions
Phase-pure white powder and even colorless single crystals of sodium yttrium ox-

Conclusions
Phase-pure white powder and even colorless single crystals of sodium yttrium oxosulfate monohydrate NaY[SO4]2 • H2O could be synthesized hydrothermally from a mix-   [69,70], IR data measured in this work.

Conclusions
Phase-pure white powder and even colorless single crystals of sodium yttrium oxosulfate monohydrate NaY  [69,70], IR data measured in this work.

Conclusions
Phase-pure white powder and even colorless single crystals of sodium yttrium oxosulfate monohydrate NaY[SO 4 ] 2 · H 2 O could be synthesized hydrothermally from a mixture of Na 2 [SO 4 ] and Y 2 [SO 4 ] 3 · 8 H 2 O in demineralized water. The anhydrate NaY[SO 4 ] 2 was obtained by thermal decomposition at temperatures above 180 • C and is stable up to 800 • C. While the trigonal crystal structure of NaY[SO 4 ] 2 · H 2 O was solved from singlecrystal X-ray diffraction data in space group P3 2 21, the monoclinic crystal structure of NaY[SO 4 ] 2 was refined with Rietveld methods from powder X-ray diffraction data in space group P2 1 /m. The Na + cations are coordinated by eight oxygen atoms from six tetrahedral [SO 4 ] 2− anions in both compounds and the coordination numbers of the Y 3+ cations in the hydrate amount to nine (eight oxygen atoms from six [SO 4 ] 2− units plus one from a water molecule) and eight again in the anhydrate (eight oxygen atoms from six [SO 4 ] 2anions). Both compounds suit as red-emitting luminescent materials, if doped with 0.5 % Eu 3+ , as shown by luminescence spectroscopy, but the anhydrate NaY[SO 4 ] 2 :Eu 3+ exhibits an almost twenty times higher quantum efficiency than the monohydrate NaY[SO 4 ] 2 · H 2 O:Eu 3+ owing to the water of hydration, which works as a vibrational quencher. The almost perfect monoexponential decay curves of the anhydrate NaY[SO 4 ] 2 :Eu 3+ and thus the lack of afterglow also prove the presence of a material with high quality, i.e., a low defect density.
Supplementary Materials: The Supplementary Material contains PXRD data from a sample after thermal treatment from 1000 • C ( Figure S1 and S2) and 1400 • C ( Figure S3) after the thermogravimetry experiment. They are available online at https://www.mdpi.com/article/10.3390/cryst11060575/s1.
Author Contributions: C.B. synthesized the pure and the Eu 3+ -doped compounds, which are described here, and measured their IR and Raman spectra. C.B. and T.S. solved the crystal structures of both compounds. T.S. contributed the reagents, the materials, the scientific equipment, and the infrastructure for synthesis, IR, and Raman spectroscopy. D.E. and T.J. measured and interpreted the Eu 3+ luminescence of both doped compounds. The paper was written by all authors. All authors have read and agreed to the published version of the manuscript.