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

Abstract

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–O_s = 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–O_w = 238 pm) in the hydrate case. In both compounds, the Na⁺ cations are atoms (d(Na–O_s = 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 τ_1/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.

1. Introduction

Eu³⁺-doped luminescence materials based on complex oxides are very important in application [1,2] and show a red emission with typical ⁵D₀ → ⁷F_J transitions between 610 and 620 nm [3]. They could be prepared on “classic” solid-state routes at high temperatures, as has been done for the examples of Y₂[MoO₄]₃:Eu³⁺ and Y₂[MoO₄]₂[Mo₂O₇]:Eu³⁺ [4], GdSb₂O₄Br:Eu³⁺ [5], as well as YNbO₄:Eu³⁺ and YTaO₄:Eu³⁺ [6]. Another energy-saving synthesis route to get Eu³⁺-doped luminescence materials without heating uses wet synthesis strategies. For example the Eu³⁺-doped xenotime-type yttrium oxoarsenate Y[AsO₄]:Eu³⁺ [7], the oxophosphate Y[PO₄]:Eu³⁺ [8], and the oxocarbonate Y₂[CO₃]₃:Eu³⁺ · n H₂O [9] were synthesized following a wet route.

With NaCe[SO₄]₂ ∙ H₂O (trigonal, P3₁21), Lindgren reported for the first time in 1977 the crystal structure of a sodium rare-earth metal oxosulfate monohydrate yielded from a wet synthesis by adding Ce[OH]₃ and Na₂[SO₄] to aqueous sulfuric acid (H₂SO₄) and heating it to 230 °C for 7 days [10]. In later works of other groups, the crystal structure of the sodium rare-earth (RE) metal oxosulfate monohydrates NaRE[SO₄]₂ ∙ H₂O for the elements RE = La—Nd and Sm—Dy was solved either in space group P3₁21 (no. 152) or its enantiomorphic analog P3₂21 (no. 154). The samples were produced by different syntheses routes [11,12,13,14,15,16,17,18] and an Indian paper from 1989 deals with these double sulfates of trivalent plutonium, as well as the rare-earth metals RE = La—Nd, Sm—Yb, and Y, but does not give some detailed crystallographic information except for the space group derived from powder X-ray diffraction data [19].

By changing the alkali metal sodium to the next bigger one, potassium namely, KLa[SO₄]₂ ∙ H₂O emerges as the only known alkali-metal rare-earth metal oxosulfate monohydrate with the mentioned trigonal structure [20]. For the smaller rare-earth metals (RE = Ce—Nd, Sm—Dy), the potassium-containing oxosulfate monohydrates KRE[SO₄]₂ ∙ H₂O crystallize monoclinically in space group P2₁/c [20,21,22,23] in analogy to the isotypic rubidium compounds RbRE[SO₄]₂ ∙ H₂O with RE = Ce, Gd, Ho and Yb [24,25,26]. For silver instead of an alkali-metal cation also AgRE[SO₄]₂ ∙ H₂O representatives with the crystal structure of NaCe[SO₄]₂ ∙ H₂O were found [27] and by the exchange of the rare-earth metal cation with trivalent bismuth, its oxosulfate monohydrate NaBi[SO₄]₂ ∙ H₂O [28] shows the same trigonal structure as the related rare-earth metal compounds NaRE[SO₄]₂ ∙ H₂O.

In 2006, the photoluminescence spectrum of NaEu[SO₄]₂ ∙ H₂O (excited at λ = 393 nm) [11] and in 2016 analogous spectra of NaTb[SO₄]₂ ∙ H₂O (excited at λ = 320 nm) and NaDy[SO₄]₂ ∙ H₂O (excited at λ = 387 nm) were measured at room temperature [12]. In 2011, Ce³⁺- and Tb³⁺-doped samples of NaY[SO₄]₂ ∙ H₂O were the subject of a luminescence investigation [29]. Moreover, in 2015, the sodium rare-earth metal oxosulfate monohydrates NaRE[SO₄]₂ ∙ H₂O with RE = La, Nd, and Gd could be successfully tested as heterogeneous redox catalysts for the selective oxidation of organic sulfides [13]. It is worth mentioning that NaY[SO₄]₂ · H₂O even occurs as a mineral with the name chinleite-(Y) [30], naturally containing all the lanthanoids with roughly the same size as yttrium. The crystallographic data from a structure refinement in space group P3₂21 have never been deposited at a common database, however.

For the anhydrous sodium rare-earth metal oxosulfates NaRE[SO₄]₂, their monoclinic crystal structure was solved in space group P2₁/m for RE = Er [31] and Tm [32] and the triclinic one in space group P$\overline{1}$ for RE = La [33] and Nd [31]. For trivalent gold instead of RE³⁺ cations, the monoclinic crystal structure of NaAu[SO₄]₂ was described in space group P2₁/n [34], but Au³⁺ in square planar oxygen coordination causes marked topological differences. Not so different from the Na⁺ analogs, for triclinic AgEu[SO₄]₂ (space group: P$\overline{1}$) with Ag⁺ in eightfold oxygen coordination, its Eu³⁺ bulk luminescence was also measured very recently [35].

In the following contribution, we report on the preparation of NaY[SO₄]₂ · H₂O via wet synthesis, its trigonal crystal structure, and the red luminescence of Eu³⁺-doped samples. After thermal decomposition, we obtained its monoclinic anhydrate NaY[SO₄]₂, which shows an even stronger red luminescence, when Eu³⁺-doped.

2. Materials and Methods

2.1. Synthesis

Sodium yttrium oxosulfate monohydrate NaY[SO₄]₂ ∙ H₂O was obtained from a wet synthesis by adding 6.6 mmol Na₂[SO₄] (ChemPur, 99.9%) and 5.5 mmol Y₂[SO₄]₃ ∙ 8 H₂O, which means an excess of Na₂[SO₄], to about 4 ml demineralized water and heated the obtained wet powder to 190 °C in a 25 ml Teflon autoclave overnight, with a yield only limited by the solubility of the monohydrate. Thus, the yield was about ²/₃ of the theoretical possible quantity. It could be increased by evaporating the water, but the change of contamination with Y₂[SO₄]₃ ∙ 8 H₂O becomes higher then. By slowly cooling the solution down (5 °C per 1 h), single crystals in a size up to 0.3 mm edge length (Figure 1) of the water-soluble colorless compound NaY[SO₄]₂ ∙ H₂O could be isolated (Equation (1)) and washed with ethanol (Brüggemann, denaturized with petrol ether). The starting material Y₂[SO₄]₃ ∙ 8 H₂O was synthesized by evaporating a solution of Y₂O₃ (ChemPur, 99.9%) in 96% sulfuric acid H₂SO₄ (Scharr, pure) according to Equation (2).

The anhydrous oxosulfate NaY[SO₄]₂ can be obtained by heating NaY[SO₄]₂ ∙ H₂O 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 Eu³⁺-doped sample of NaY[SO₄]₂ ∙ H₂O (0.5% Eu instead of Y) was produced by adding Eu₂[SO₄]₃ ∙ 8 H₂O (synthesis analogous to Y₂[SO₄]₃ ∙ 8 H₂O with Eu₂O₃ (ChemPur, 99.9%) instead of Y₂O₃) to the process, which is described in Equation (1), and NaY[SO₄]₂:Eu³⁺ has been prepared by draining the doped sample at 550 °C in air.

Na₂[SO₄] + Y₂[SO₄]₃ ∙ 8 H₂O → 2 NaY[SO₄]₂ ∙ H₂O + 7 H₂O

(1)

RE₂O₃ + 3 H₂SO₄ + 5 H₂O → RE₂[SO₄]₃ ∙ 8 H₂O (RE = Y and Eu)

(2)

NaY[SO₄]₂ ∙ H₂O → NaY[SO₄]₂ + H₂O ↑

(3)

2.2. 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[SO₄]₂ ∙ H₂O (CSD-2016596) in the trigonal space group P3₂21 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[SO₄]₂ (CSD-2072719) was measured from 2ϴ = 8–110° for solving its crystal structure in the NaEr[SO₄]₂-type arrangement [31] with the program FULLPROF [39,40]. The measured powder X-ray diffraction pattern of NaY[SO₄]₂ ∙ H₂O can be seen in Figure 2 (top) and the measured PXRD pattern together with the difference plot of the Rietveld refinement for NaY[SO₄]₂ is shown in Figure 2 (bottom).

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.

While all the atomic displacement parameters of NaY[SO₄]₂ ∙ H₂O could be refined anisotropically based on single-crystal X-ray diffraction data, the atomic displacement parameters of NaY[SO₄]₂ were only treated isotropically with Rietveld refinement based on PXRD data.

2.3. Thermal Analysis

Thermal analysis (thermogravimetry) was performed with about 36 mg of a NaY[SO₄]₂ ∙ H₂O 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.

2.4. 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[SO₄] 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[SO₄]₂:Eu³⁺ 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]:

$$LE\left(\mathrm{lm}/\mathrm{W}\right)=683 (\mathrm{lm}/\mathrm{W})\cdot \frac{{{\displaystyle \int }}_{380\mathrm{nm}}^{780\mathrm{nm}}I\left(\lambda \right)V\left(\lambda \right)d\lambda }{{{\displaystyle \int }}_{380\mathrm{nm}}^{780\mathrm{nm}}I\left(\lambda \right)d\lambda }$$

2.5. IR and Raman Spectra

Infrared spectra for powder samples of NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]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.

3. Results and Discussion

3.1. Structure Refinement and Description of NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂

The most relevant crystallographic data of the wet synthesized NaY[SO₄]₂ ∙ H₂O compared to its anhydrate NaY[SO₄]₂ are shown in

Table 1. Crystallographic data of NaY[SO₄]₂ ∙ H₂O (left) and NaY[SO₄]₂ (right).

Compound	NaY[SO4]2 ∙ H2O		NaY[SO4]2
Crystal system	trigonal		monoclinic
Space group	P3221 (no. 154)		P21/m (no. 11)
Lattice parameters,
a [pm]	682.24(5)		467.697(5)
b [pm]	= a		686.380(6)
c [pm]	1270.65(9)		956.597(9)
β [°]	90		96.8079(5)
Number of formula units, Z	3		2
Unit-cell volume, Vuc [nm³]	0.51219(4)		0.304919(5)
Molar volume, Vm [cm3 ∙ mol−1]	102.81		91.81
Calculated density, Dx [g ∙ cm−3]	3.132		3.311
Diffraction method	single crystal		powder
Instrument	κ-CCD		Stadi-P (transmission)
Radiation	Mo-Kα, λ = 71.07 pm		Cu-Kα, λ = 154.06 pm
Structure resolution and refinement	SHELX-97		FULLPROF
Range in ±h, ±k, ±l	8, 8, 16		4, 7, 10
Range of 2ϴ [°]	3–55		8–110
Absorption coefficient, μ [mm−1]	19.25		−
Extinction coefficient, g	0.0174(15)		−
Reflections collected	8159		438
and unique	786		−
Rint / Rσ	0.080/0.036		−
R1 / wR2 for all reflections	0.031/0.070		−
Goodness of Fit (GooF)	1.074		−
Residual e— density (max. / min.)	0.60 and −0.48		−
Flack-x parameter	−0.021(9)		−
Rp	−		4.67
Rwp	−		7.52
Rexp	−		4.33
χ2	−		3.02
CSD number	2016596		2072719

. The given lattice parameters of NaY[SO₄]₂ ∙ H₂O stems from single-crystal data, while its lattice parameters from PXRD experiments amount to a = 682.82(3) pm and c = 1270.77(6) pm (c/a = 1.861).

Table 2. Fractional atomic coordinates, site symmetry and U values * of NaY[SO₄]₂ ∙ H₂O (top) and NaY[SO₄]₂ (bottom).

Atom	Wyckoff Site	Symmetry	x/a	y/b	z/c	U/pm2
Na	3b	.2.	0.5299(3)	0	1/6	211(5)
Y	3a	.2.	0	0.56341(8)	1/3	145(2)
S	6c	1	0.9864(2)	0.5437(2)	0.09243(6)	134(2)
O1	6c	1	0.1273(5)	0.5055(5)	0.0180(2)	217(7)
O2	6c	1	0.8273(5)	0.5829(5)	0.0316(2)	210(7)
O3	6c	1	0.8677(5)	0.3517(5)	0.1655(2)	192(7)
O4	6c	1	0.1249(5)	0.7408(5)	0.1610(2)	196(7)
O5w	3a	.2.	0	0.9123(8)	1/3	369(14)
H	6c	1	0.063(11)	0.957(11)	0.042(4)	554(36)
Na	2e	m	0.6289(11)	1/4	0.3506(4)	195(12)
Y	2e	m	0.6536(3)	1/4	0.82110(12)	167(3)
S1	2e	m	0.1619(7)	1/4	0.5875(3)	163(9)
S2	2e	m	0.1407(6)	1/4	0.0715(3)	183(9)
O1	2e	m	0.8254(13)	1/4	0.0738(6)	114(18)
O2	2e	m	0.2317(13)	1/4	0.9259(6)	105(17)
O3	4f	1	0.3075(10)	0.0730(6)	0.6574(4)	177(14)
O4	4f	1	0.2628(10)	0.0699(6)	0.1470(4)	176(13)
O5	2e	m	0.8757(14)	1/4	0.6311(6)	119(18)
O6	2e	m	0.1881(13)	1/4	0.4401(6)	106(18)

* U values for NaY[SO₄]₂ ∙ H₂O: U_eq = ¹/₃ [U₃₃ + ⁴/₃ (U₁₁ + U₂₂ − U₁₂)] [44], but for all atoms of NaY[SO₄]₂ and H of NaY[SO₄]₂ ∙ H₂O: U_iso.

shows the fractional atomic coordinates with the site symmetry for all atoms and U_eq or U_iso values of NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂.

While NaY[SO₄]₂ ∙ H₂O crystallizes in the trigonal space group P3₂21 (no. 154) with a = 682.24(5) pm, and c = 1270.65(9) pm (c/a = 1.862) for Z = 3, NaY[SO₄]₂ adopts the monoclinic space group P2₁/m (no. 11) with a = 467.697(5) pm, b = 686.380(6) pm, c = 956.597(10) pm, and β = 96.8079(5)° for Z = 2. The b-axes of both compounds differ by only 0.6% and the c-axis of the monohydrate is about ⁴/₃ of the one of the anhydrate. While in the hydrate monolayers of Na⁺ and Y³⁺ cations take turns along [001], in the anhydrate double layers of each Na⁺ and Y³⁺ alternate along [001]. Extended unit cells of NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂ can be seen in Figure 3.

In NaY[SO₄]₂ ∙ H₂O, the Y³⁺ cations are coordinated by nine oxygen atoms (eight from oxosulfate anions (d(Y–O) = 237–248 pm) and one from a water molecule (d(Y–O5w) = 238 pm). Only eight oxygen atoms covalently bonded to sulfur in [SO₄]^2–, and units occur as Y³⁺ coordination sphere (d(Y–O) = 220–277 pm) in NaY[SO₄]₂.

Y³⁺ in NaY[SO₄]₂ ∙ H₂O resides on the Wyckoff site 3a with C₂ symmetry (Figure 4, left), whereas Y³⁺ in NaY[SO₄]₂ occupies the 2e position on a mirror plane (Figure 4, right).

In Y₂[SO₄]₃ ∙ 8 H₂O [45], the unique Y³⁺ 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, Y³⁺ is surrounded octahedrally by only six oxygen atoms from oxosulfate groups with distances between 220 and 224 pm [46]. While Y³⁺ is coordinated by just one oxygen atom per [SO₄]^2– anion in both Y₂[SO₄]₃ ∙ 8 H₂O [45] and Y₂[SO₄]₃ [46], the same is observed in NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂, but now with two oxygen atoms of the same oxosulfate unit. In Y₂[SO₄]₃ [46], NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂ six [SO₄]^2– anions coordinate the Y³⁺ cations, while in Y₂[SO₄]₃ ∙ 8 H₂O [45] there are only four of them. Compounds of the type ARE[SO₄]₂ ∙ H₂O with A = Na crystallize trigonally in space group P3₂21 (or P3₁21) [10,11,12,13,14,15,16,17,18], but monoclinically in space group P2₁/c with A = K for RE = Ce—Nd, Sm—Dy [20,21,22,23]. The water molecule in NaY[SO₄]₂ ∙ H₂O is only coordinated to yttrium, whereas in the KRE[SO₄]₂ ∙ H₂O examples [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 P$\overline{1}$ 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 RE³⁺ 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.

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 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 [SO₄]^2– units in the monoclinic cases [49,50]. In the orthorhombic salt Na₂[SO₄], the sodium cations show six oxygen atoms from five oxosulfate groups as next neighbors [51], while in its decahydrate Na₂[SO₄] ∙ 10 H₂O, 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 Y³⁺, Na^+, and S⁶⁺, while in NaY[SO₄]₂ O2 and O6 differ from this scheme since O2 is coordinated by one S⁶⁺ and two Y³⁺ and O6 by one S⁶⁺ and two Na⁺ cations. Even O5w has one Y³⁺ 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. Selected interatomic distances (d/pm) in the crystal structures of NaY[SO₄]₂ ∙ H₂O (left) and NaY[SO₄]₂ (right).

NaY[SO4]2 ∙ H2O			NaY[SO4]2
d(Y–O2)	(1×)	236.7(3)	d(Y–O5)	(1×)	219.8(7)
d(Y–O2)	(1×)	239.7(4)	d(Y–O4)	(2×)	224.5(4)
d(Y–O5W)	(1×)	238.0(3)	d(Y–O2)	(1×)	231.7(6)
d(Y–O1)	(2×)	239.1(3)	d(Y–O3)	(2×)	243.9(4)
d(Y–O4)	(2×)	224.0(3)	d(Y–O1)	(1×)	245.5(6)
d(Y–O3)	(2×)	247.9(3)	d(Y–O2)	(1×)	277.0(6)
$\overline{d}$(Y–O)	(C.N. = 9)	241.5	$\overline{d}$(Y–O)	(C.N. = 8)	238.8
d(Na–O3)	(2×)	235.4(4)	d(Na–O3)	(2×)	223.9(4)
d(Na–O4)	(2×)	242.5(4)	d(Na–O6)	(1×)	232.4(8)
d(Na–O1)	(2×)	253.6(3)	d(Na–O6)	(1×)	265.4(8)
d(Na–O2)	(2×)	287.9(3)	d(Na–O4)	(2×)	273.2(5)
$\overline{d}$(Na–O)	(C.N. = 8)	254.9	d(Na–O5)	(1×)	279.3(7)
			d(Na–O1)	(1×)	290.4(7)
			$\overline{d}$(Na–O)	(C.N. = 8)	257.7
d(S–O1)	(1×)	146.2(3)
d(S–O2)	(1×)	146.2(4)	d(S1–O6)	(1×)	141.0(7)
d(S–O3)	(1×)	147.4(3)	d(S1–O5)	(1×)	144.9(8)
d(S–O4)	(1×)	148.0(3)	d(S1–O3)	(2×)	151.0(4)
$\overline{d}$(S–O)	(C.N. = 4)	147.2	$\overline{d}$(S1–O)	(C.N. = 4)	147.4
			d(S2–O1)	(1×)	147.7(7)
			d(S2–O2)	(1×)	150.4(7)
			d(S2–O4)	(2×)	150.9(4)
			$\overline{d}$(S2–O)	(C.N. = 4)	150.0

.

For confirmation of the Na⁺ and Y³⁺ sites in NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂, bond-valence calculations were carried out with the parameters used by Brese and O’Keeffe [53]. With calculated charges of 3.06 in NaY[SO₄]₂ ∙ H₂O and 3.19 in NaY[SO₄]₂ for the Y³⁺ sites next to 1.20 in NaY[SO₄]₂ ∙ H₂O and 1.23 in NaY[SO₄]₂ for the Na⁺ sites, their positions can just be confirmed. More details of these calculations can be seen in

Table 4. Results of the bond-valence calculations for NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂.

NaY[SO4]2 ∙ H2O
for Y	O2	O2’	O5	O1	O1’	O4	O4’	O3	O3’
d(Y–O) [pm]	236.68	236.70	238.03	239.12	239.13	243.98	244.06	247.86	247.93	∑(vij)
vij	0.385	0.385	0.372	0.361	0.361	0.316	0.316	0.285	0.284	3.065
for Na	O3	O3’	O4	O4’	O1	O1’	O2	O2’
d(Na–O) [pm]	235.35	235.35	242.50	242.50	253.58	253.66	287.92	287.99		∑(vij)
vij	0.224	0.224	0.185	0.185	0.137	0.137	0.054	0.054		1.199
for S	O1	O2	O3	O4			for H		O5w
d(S–O) [pm]	101.46	101.46	101.47	101.48	∑(vij)		d(H–O) [pm]		97.86
vij	1.550	1.549	1.500	1.477	6.076			vij	0.926
NaY[SO4]2
for Y	O5	O4	O4	O2	O3	O3	O1	O2
d(Y–O) [pm]	219.77	224.48	224.48	231.68	243.87	243.87	245.53	277.00		∑(vij)
vij	0.609	0.536	0.536	0.441	0.317	0.317	0.303	0.130		3.189
for Na	O3	O3’	O6	O6’	O4	O4’	O5	O1
d(Na–O) [pm]	223.94	223.94	232.42	265.37	272.18	273.18	279.25	290.39		∑(vij)
vij	0.305	0.305	0.242	0.100	0.083	0.081	0.068	0.051		1.234
for S1	O6	O5	O3 (2×)		for S2		O1	O2	O4 (2×)
d(S1–O) [pm]	142.97	144.85	150.96	∑(vij)	d(S2–O) [pm]		147.74	150.39	150.93	∑(vij)
vij	1.691	1.607	1.362	6.022		vij	1.486	1.383	1.363	5.597
Rij constant from [53] for			Y	Na	S	H
	distance to O		2.014	1.80	1.624	0.95	Å

. The bond-valence equation for the calculation of the charge given by Brese and O’Keeffe [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.

The motifs of mutual adjunction for the atoms in both title compounds NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂ can be seen in

Table 5. Motifs of mutual adjunction for NaY[SO₄]₂ ∙ H₂O (top) and NaY[SO₄]₂ (bottom).

NaY[SO4]2 ∙ H2O	O1	O2	O3	O4	O5w		C.N.
Y	2/1	2/1	2/1	2/1	1/1		9
Na	2/1	2/1	2/1	2/1	0/0		8
S	1/1	1/1	1/1	1/1	0/0		4
H	0/0	0/0	0/0	0/0	1/2		1
C.N.	3	3	3	3	3
NaY[SO4]2	O1	O2	O3	O4	O5	O6	C.N.
Y	1/1	2/2	2/1	2/1	1/1	0/0	8
Na	1/1	0/0	2/1	2/1	1/1	2/2	8
S1	0/0	0/0	2/1	0/0	1/1	1/1	4
S2	1/1	1/1	0/0	2/1	0/0	0/0	4
C.N.	3	3	3	3	3	3

.

We became aware of a competing structure refinement for trigonal NaY[SO₄]₂ · H₂O (a = 681.91(3) pm, c = 1270.35(11) pm, c/a = 1.863) in space group P3₁21 that was already in the progress of publication [54], simultaneous to our activities writing this article. The lower CSD deposition number (ours for NaY[SO₄]₂ · H₂O in space group P3₂21: 2016596 versus the Chinese competitor one for NaY[SO₄]₂ · H₂O in space group P3₁21: 2058909) should grant us a priority, despite the almost identical results in both papers from the year 2021.

3.2. Thermal Analysis

A thermogravimetrical curve for the decomposition of NaY[SO₄]₂ ∙ H₂O between 25 and 1400 °C is depicted in Figure 9.

The first mass-loss at about 180 °C with 5.6% represents the release of water and the transformation from NaY[SO₄]₂ ∙ H₂O (100% mass; M = 322.036 g/mol) to NaY[SO₄]₂ (94.4% mass; M = 304.021 g/mol). The second decomposition leads to a mixture of Y₂O₂[SO₄] [55] with the crystal structure of monoclinic La₂O₂[SO₄] [56] and Eu₂O₂[SO₄] [57] or orthorhombic Nd₂O₂[SO₄] [58] together with Na₂[SO₄] [51,59], confirmed by powder X-ray diffraction experiments (Figures S1 and S2 in the Supplementary Information). For Y₂O₂[SO₄], there are no known or other good crystal-structure data available, so there are differences in intensity and position, but the final decomposition step leads to a mixture of cubic Y₂O₃ with a bixbyite-type structure [60] and orthorhombic Na₂[SO₄] [51,59] (Figure S3). The TG curve (Figure 9) appears to be similar to that of NaRE[SO₄]₂ ∙ H₂O 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 RE³⁺-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).

While the TG curve (Figure 9) shows a phase transformation from NaY[SO₄]₂ ∙ H₂O to NaY[SO₄]₂ at 180 °C, the temperature-depending PXRD indicates the anhydrous compound for the first time at 350 °C. At 550 °C the water-containing compound could not be detected anymore. The XRD intensities became lower again with rising temperatures and suggest a starting decomposition of NaY[SO₄]₂ to Y₂O₂[SO₄] and Na₂[SO₄]. The reflection at 12.8° resulted from the X-ray powder-diffractometer setting.

3.3. Luminescence-Spectroscopic Properties

Eu³⁺-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 Eu³⁺, which is a typical energetic position of the LMCT process of Eu³⁺ 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 Eu³⁺ absorption lines originating from the ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂ transitions could be observed in the ranges of 395–397 nm and 450–470 nm, respectively [63].

Temperature-dependent excitation spectra of NaY[SO₄]₂ · H₂O:Eu³⁺ and NaY[SO₄]₂:Eu³⁺ reveal the typical intraconfigurational 4f–4f transitions of Eu³⁺ between 280 and 550 nm [63,64] and the position of the LMCT of Eu³⁺ 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[SO₄]₂ ∙ H₂O:Eu³⁺, since a closer look at the UV-A range revealed a distinct change of the pattern of ⁷F₀ → ⁵L₆ (390–405 nm) and ⁷F₀ → ⁵L₈ + ⁵G_J + ⁵L₉ + ⁵L₁₀ (J = 2–6) (373–387 nm) transitions [65]. The thermal population of the ⁷F₁ level could explain some changes in the excitation line pattern. However, the shift and broadening of the most intense line of the ⁷F₀ → ⁵L₆ 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:Eu³⁺ to NaY[SO₄]₂:Eu³⁺ in good accordance with the results from thermal gravimetry (Figure 9). Temperature-dependent emission spectra of NaY[SO₄]₂ ∙ H₂O:Eu³⁺ and NaY[SO₄]₂:Eu³⁺ are shown in Figure 14.

The emission spectra of Eu³⁺-comprising materials consisted of the orange allowed magnetic-dipole (MD) transition ⁵D₀ → ⁷F₁, the red parity-forbidden electric-dipole (ED) transition ⁵D₀ → ⁷F₂, and further line multiplets in the deep red spectral range around 650 and 695 nm due to the ED transitions ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄. For light sources and emissive displays, the emission spectrum should consist mainly of emission lines resulting from the ⁵D₀ → ⁷F₂ transitions [66,67]. This means that the Eu³⁺ 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 ⁵D₀ → ⁷F₂ transition is hypersensitive and small deviations of the inversion symmetry strongly enhance the probability of the ⁵D₀ → ⁷F₂ transitions. The intensity of the strongly forbidden transition ⁵D₀ → ⁷F₀ is known to correlate with the linear terms of the crystal-field parameter and polarizability of the Eu³⁺ cation [67].

However, the emission spectrum of NaY[SO₄]₂ ∙ H₂O:Eu³⁺ upon 395 nm excitation revealed the typical emission line pattern between 580 and 720 nm due to the ⁵D₀ → ⁷F_J (J = 0–4) transitions of Eu³⁺ [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 Eu³⁺ luminescence [69].

As already observed for the excitation spectra, the temperature-dependent emission spectra of NaY[SO₄]₂ ∙ H₂O:Eu³⁺ showed a distinct change once the temperature exceeded 400 K, resulting in the increase of intensity and the width of the ⁵D₀ → F₁, ⁵D₀ → ⁷F₂, and ⁵D₀ → ⁷F₄ transitions, as well as the appearance of the ⁵D₀ → ⁷F₀ transition, which was absent at room temperature. This change again points to a phase transition, i.e., the transformation of NaY[SO₄]₂ ∙ H₂O:Eu³⁺ to NaY[SO₄]₂:Eu³⁺, which goes along with an increase of the crystal-field strength causing a larger energetic spread of the Stark components of the above mentioned ⁵D₀ → ⁷F_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[SO₄]₂:Eu³⁺ 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:Eu³⁺ 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 ⁵D₀ → ⁷F₂ /⁵D₀ → ⁷F₁ [63] or by the reduction of the covalency related to the ⁵D₀ → ⁷F₄/⁵D₀ → ⁷F_J ratio [3]. However, both effects are in line with a thermal expansion of the crystals and the Eu³⁺ site causes a decrease of the covalent interaction between Eu³⁺ and oxygen and an increase of the local symmetry.

Noteworthy were the intensities of the temperature-dependent emission spectra of NaY[SO₄]₂ ∙ H₂O:Eu³⁺ 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:Eu³⁺ upon increasing the temperature.

In contrast, the temperature-dependent emission spectra of NaY[SO₄]₂:Eu³⁺ itself show a typical decrease of the intensity or quantum yield of Eu³⁺ phosphors with increasing temperature [63].

Finally, we investigated the time-dependent luminescence (Figure 17) of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ at 617 nm upon 395 nm excitation of NaY[SO₄]₂ ∙ H₂O:Eu³⁺ and NaY[SO₄]₂:Eu³⁺. As discussed before, NaY[SO₄]₂ ∙ H₂O:Eu³⁺ 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[SO₄]₂:Eu³⁺ 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 Eu³⁺ photoluminescence is a minor issue.

3.4. IR and Raman Studies of NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂

The Raman and IR spectra of NaY[SO₄]₂ ∙ H₂O and NaY[SO₄]₂ are shown together with those of Y₂[SO₄]₃ ∙ 8 H₂O and Na₂[SO₄] in Figure 18 and the values are given in

Table 6. Raman (black) and IR vibration values (blue) of NaY[SO₄]₂ and NaY[SO₄]₂ ∙ H₂O as compared to those of Y₂[SO₄]₃ ∙ 8 H₂O, Y₂[SO₄]₃ [69], and Na₂[SO₄] (thenardite) [70].

$\tilde{\mathit{\nu }} \left[{\mathbf{cm}}^{-1}\right]$	NaY[SO4]2	NaY[SO4]2 ∙ H2O	Y2[SO4]3 ∙ 8 H2O	Y2[SO4]3 *	Na2[SO4] (thenardite) *
[SO4]2–
δs	413, 479	429, 492	442, 450, 467	452, 484, 504	451, 466
δas	608, 629, 669   667, 685	628, 669   600, 626, 660	618   638, 652, 688, 743	609, 654	621, 632, 647   609, 634, 668
νs	1015, 1046 1007, 1010, 1068	1020 1012, 1032, 1092	1015 1002, 1080, 1032	1013	933
νas	1080, 1085, 1140, 1172 1120, 1140, 1289	1145, 1168   1135, 1165	1080, 1088, 1112, 1146 1132	1122, 1145, 1184	1102, 1129, 1152   1090
H2O
νas,s		3536, 3592	3229, 3348, 3467
δ		1606	1640

* Raman data of Y₂[SO₄]₃ and Na₂[SO₄] from literature [69,70], IR data measured in this work.

compared to the literature data for Y₂[SO₄]₃ [69] and Na₂[SO₄] (thenardite) [70]. The vibration at about 2300 cm^–1 represents CO₂ in the laboratory environment. While the ideal [SO₄]^2– anion with T_d symmetry should only have four visible vibration bands (ν_as and δ_as: IR and Raman active, ν_s and δ_s: only Raman active), in the measured solid-state samples there were more bands measured. This was because of the no longer ideal [SO₄]^2– units and their considerable symmetry reduction.

4. 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 mixture of Na₂[SO₄] and Y₂[SO₄]₃ ∙ 8 H₂O in demineralized water. The anhydrate NaY[SO₄]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[SO₄]₂ ∙ H₂O was solved from single-crystal X-ray diffraction data in space group P3₂21, the monoclinic crystal structure of NaY[SO₄]₂ was refined with Rietveld methods from powder X-ray diffraction data in space group P2₁/m. The Na⁺ cations are coordinated by eight oxygen atoms from six tetrahedral [SO₄]²⁻ anions in both compounds and the coordination numbers of the Y³⁺ cations in the hydrate amount to nine (eight oxygen atoms from six [SO₄]²⁻ units plus one from a water molecule) and eight again in the anhydrate (eight oxygen atoms from six [SO₄]^2— anions). Both compounds suit as red-emitting luminescent materials, if doped with 0.5 % Eu³⁺, as shown by luminescence spectroscopy, but the anhydrate NaY[SO₄]₂:Eu³⁺ exhibits an almost twenty times higher quantum efficiency than the monohydrate NaY[SO₄]₂ ∙ H₂O:Eu³⁺ owing to the water of hydration, which works as a vibrational quencher. The almost perfect monoexponential decay curves of the anhydrate NaY[SO₄]₂:Eu³⁺ and thus the lack of afterglow also prove the presence of a material with high quality, i.e., a low defect density.