Y₅F₃[AsO₃]₄ and Y₅Cl₃[AsO₃]₄: Two Non-Isostructural Yttrium Halide Oxoarsenates(III) and Their Potential as Hosts for Luminescent Eu³⁺- and Tb³⁺-Doping

Abstract

Y₅F₃[AsO₃]₄ crystallizes needle-shaped in the tetragonal space group P4/ncc with the lattice parameters a = 1143.80(8) pm, c = 1078.41(7) pm and c/a = 0.9428 for Z = 4. The yttrium-fluoride substructure linked via secondary contacts forms a three-dimensional network $\begin{array}{c}3\\ \infty \end{array}${[Y₅F₃]¹²⁺} and the remaining part consists of ψ¹-tetrahedral [AsO₃]³⁻ units, which leave lone-pair channels along [001]. In contrast, platelet-shaped Y₅Cl₃[AsO₃]₄ crystals adopt the monoclinic space group C2/c with the lattice parameters a = 1860.56(9) pm, b = 536.27(3) pm, c = 1639.04(8) pm and β = 105.739(3)° for Z = 4. Condensation of [(Y1,2)O₈]¹³⁻ polyhedra via four common edges each leads to fluorite-like $\begin{array}{c}2\\ \infty \end{array}$ {[(Y1,2)O $\begin{array}{c}e\\ 8/2\end{array}$ ]⁵⁻} layers spreading out parallel to the (100) plane. Their three-dimensional linkage occurs via the (Y3)³⁺ cations with their Cl⁻ ligands on the one hand and the As³⁺ cations with their lone-pairs of electrons on the other, which also form within [AsO₃]³⁻ anions lone-pair channels along [010]. Both colorless compounds can be obtained by solid-state reactions from corresponding mixtures of the binaries (Y₂O₃, As₂O₃ and YX₃ with X = F and Cl) at elevated temperatures of 825 °C, most advantageously under halide-flux assistance (CsBr for Y₅F₃[AsO₃]₄ and ZnCl₂ for Y₅Cl₃[AsO₃]₄). By replacing a few percent of YX₃ with EuX₃ or TbX₃, Eu³⁺- or Tb³⁺-doped samples are accessible, which show red or green luminescence upon excitation with ultraviolet radiation.

1. Introduction

As a consequence of the lanthanoid contraction, yttrium displays a cationic radius (r_i(Y³⁺) = 101.9 pm, Z(Y) = 39) close to the one of holmium (r_i(Ho³⁺) = 101.5 pm, Z(Ho) = 67), despite having only half the atomic mass [1]. Owing to the electronic closed-shell situation of Y³⁺ ([Kr]), yttrium(III) compounds serve as perfect hosts for getting doped with same-size luminescent cations of the lanthanoid series (e.g., Eu³⁺ or Tb³⁺), especially when they contain hard anions (O²⁻ or F⁻) to avoid quenching effects. But not only Y₂O₃ [2] and YF₃ [3] are suitable for this purpose, also mixed anionic compounds like YOF [4] are promising candidates, even when they bear soft anions (e.g., YOCl [5] or Y₂O₂S [6]). Moreover, the necessary energy-transfer processes for luminescence can be eased, when inorganic antennae are present in more complex systems, such as fluoride salts with oxoanions. These serve as energy reservoirs for incoming radiation and act with charge-transfer or electronic lone-pair activities. Most prominent examples are the oxomolybdates(VI) and oxotungstates(VI) YF[MoO₄] [7] and YF[WO₄] [8] for the first, or the oxoselenates(IV) YF[SeO₃] [9,10] and Y₃F[SeO₃]₄ [11] for the second case. Even here, the softer Cl⁻ anions can be tolerated, as the chloride oxomolybdate(VI) YCl[MoO₄] [7], the chloride oxotungstate(VI) YCl[WO₄] [12,13] and the chloride oxoantimonate(III) YClSb₂O₄ [14] might demonstrate. The idea of combining most of these prerequisites leads to investigations of the yttrium(III) halide oxoarsenates(III) Y₅X₃[AsO₃]₄ (X = F and Cl) as host materials for doping with small amounts of Eu³⁺ or Tb³⁺ in order to harvest red or green light from a colorless compound upon UV irradiation.

2. Experimental

The new yttrium(III) fluoride oxoarsenate(III) Y₅F₃[AsO₃]₄ and its doped samples could be prepared via partial metallothermic reduction. For this purpose, elemental yttrium (Y: ChemPur, 99.9%, 89.5 mg), yttrium trifluoride (YF₃: Heraeus, 99.9%, 35.6 mg) and arsenic sesquioxide (As₂O₃: Aldrich, 99.99%, 199.2 mg) were reacted within a flux of cesium bromide (CsBr: Merck, 99.9%, 800 mg) in evacuated glassy silica ampoules (Equation (1)). Europium trifluoride (EuF₃: ChemPur, 99.9%, 1.6 mg) and terbium trifluoride (TbF₃: ChemPur, 99.9%, 1.6 mg) were used as dopants, replacing aliquots of YF₃.

$$\begin{array}{l}4 \mathrm{Y}+1-x {\mathrm{YF}}_3\left(+x Ln{\mathrm{F}}_3\right)+4 {\mathrm{As}}_2{\mathrm{O}}_3\to {\mathrm{Y}}_5{\mathrm{F}}_3{\left[{\mathrm{AsO}}_3\right]}_4+4 \mathrm{As}\hfill \\ (Ln=\mathrm{Eu}\mathrm{or}\mathrm{Tb},\mathrm{flux}: \mathrm{CsBr})\hfill \end{array}$$

(1)

The ampoules were sealed under inert gas (argon), treated with a temperature program at 825 °C and brought to room temperature over several days. In this way, phase-pure powders could be obtained and only the monolithically grown single crystal of arsenic per batch had to be removed. As an example, Figure 1 shows the Rietveld refinement of an undoped powder sample of Y₅F₃[AsO₃]₄ measured with a STADI-P diffractometer (Stoe & Cie, Darmstadt, Germany) using Cu-K_α radiation (λ = 154.06 pm).

Figure 1. Rietveld refinement (black) of a powder diffractogram of undoped Y₅F₃[AsO₃]₄ (red) with the Bragg positions (green) of Y₅F₃[AsO₃]₄ and the Y_obs–Y_cal difference curve (blue).

The new yttrium(III) chloride oxoarsenate(III) Y₅Cl₃[AsO₃]₄ and its doped samples were able to be prepared by mixing yttrium sesquioxide (Y₂O₃: Merck, 99.99%, 108.3 mg), yttrium trichloride (YCl₃: ChemPur, 99.9%, 45.4 mg) and arsenic sesquioxide (As₂O₃: Aldrich, 99.99%, 94.9 mg) with a flux of zinc chloride (ZnCl₂: Merck, 99.9%, 800 mg) in evacuated glassy silica ampoules (Equation (2)). Europium trichloride (EuCl₃: Aldrich, 99.9%, 1.9 mg) and terbium trichloride (TbCl₃: ChemPur, 99.9%, 1.9 mg) were used as dopants to replace the corresponding amounts of YCl₃.

$$\begin{array}{l}{2 \mathrm{Y}}_2{\mathrm{O}}_3 + 1-\mathit{x} {\mathrm{YCl}}_3 (+\mathit{x} Ln{\mathrm{C}\mathrm{l}}_3) + 2 {\text{As}}_2{\mathrm{O}}_3\to {\mathrm{Y}}_5{\text{Cl}}_3{\left[\text{As}{\mathrm{O}}_3\right]}_4\hfill \\ (Ln=\mathrm{Eu}\mathrm{or}\mathrm{Tb},\mathrm{flux}: {\mathrm{ZnCl}}_2)\hfill \end{array}$$

(2)

The ampoules were sealed under inert gas (argon), treated with a temperature program at 825 °C and brought to room temperature over several days. In this way, also phase-pure powder samples could be obtained. As an example, Figure 2 shows the Rietveld refinement of an undoped powder of Y₅Cl₃[AsO₃]₄ measured with a Rigaku SmartLab X-ray powder diffractometer (Rigaku, Tokyo, Japan) with Cu-K_α radiation (λ = 154.06 pm).

Figure 2. Rietveld refinement (black) of a powder diffractogram of undoped Y₅Cl₃[AsO₃]₄ (red) with the Bragg positions (green) of Y₅Cl₃[AsO₃]₄ and the Y_obs–Y_cal difference curve (blue).

Suitable single crystals of needle-shaped Y₅F₃[AsO₃]₄ and platelet-shaped Y₅Cl₃[AsO₃]₄ (Figure 3) were transferred into glass capillaries (Hilgenberg, Malsfeld, Germany) and fixed with grease, then measured with Mo-K_α radiation (λ = 71.07 pm) on a κ-CCD single-crystal diffractometer (Bruker-Nonius, Karlsruhe, Germany). After numerical absorption correction with the program HABITUS, included in the X-SHAPE [15] suite, using the SHELX-97 program package [16], the resulting data sets could be solved and successfully refined with direct methods in the tetragonal space group P4/ncc for Y₅F₃[AsO₃]₄ and in the monoclinic space group C2/c for Y₅Cl₃[AsO₃]₄.

Figure 3. Brick- or needle-shaped crystals of Y₅F₃[AsO₃]₄ (left) and platelet-shaped crystal of Y₅Cl₃[AsO₃]₄ (right).

The samples were recorded on an FS920 fluorescence spectrometer (Edinburgh Instruments, Livingston, UK) at room temperature with a Xe900 continuous xenon lamp. Three scans were recorded with respect to the lamp intensity and the average is displayed in the spectra. They were plotted and standardized with respect to their intensity with Origin 2019b.

3. Results and Discussion

3.1. Crystal-Structure Description of Y₅F₃[AsO₃]₄

The new yttrium(III) fluoride oxoarsenate(III) Y₅F₃[AsO₃]₄ crystallizes needle-shaped in the tetragonal space group P4/ncc with the lattice parameters a = 1143.80(8) pm, c = 1078.41(7) pm and c/a = 0.943 for Z = 4. As expected and as can be seen from the Shannon radii [1], it should assume a unit-cell size between those of Dy₅F₃[AsO₃]₄ (a = 1148.34(8) pm, c = 1082.69(7) pm, c/a = 0.943) [17] and Ho₅F₃[AsO₃]₄ (a = 1143.26(8) pm, c = 1078.14(7) pm, c/a = 0.943) [17] in terms of the unit-cell dimensions.

There are three different cationic sites present in this structure, two of them belong to yttrium (4c for Y1 and 16g for Y2) and one to arsenic (16g). In contrast, the anions have five different sites, three of which belong to oxygen (all at 16g) and two to fluorine (4c for F1 and 8f for F2). The (Y1)³⁺ cation centers a square prism of oxygen (d(Y1–O) = 243–249 pm), which is capped by two fluoride anions [(Y1)O₈(F1)(F1′)]¹⁵⁻. One F⁻ anion comes close to (Y1)³⁺ (d(Y1–F1) = 240 pm), while the second one is much further away (d(Y1⋯F1′) = 300 pm) and can therefore only be regarded as a secondary-contact ligand (Figure 4, left). In contrast, the (Y2)³⁺ cation forms a bicapped trigonal prism [(Y2)O₆F₂]¹¹⁻ with six oxygen atoms and two fluoride anions without secondary contacts (Figure 4, mid). However, the range of bond lengths is significantly wider (d(Y2–F) = 221–261 pm, d(Y2–O) = 227–250 pm). These distances are still within the expected range, however, especially when compared with Y₇O₆F₉ [18,19], where yttrium-fluorine distances from 218 to 260 pm (plus 300 pm) are present and secondary contacts occur here as well. The yttrium-oxygen distances of 220–268 pm also fall into a similar interval. The As³⁺ cation forms an isolated ψ¹-tetrahedron [AsO₃]³⁻ with three oxygen atoms (Figure 4, right), showing arsenic-oxygen distances in the range from 176 to 181 pm, which are quite typical, especially when compared with the crystalline As₂O₃ modifications (claudetite-I: 172–181 pm [20], claudetite-II: 177–182 pm [21], arsenolite: 179 pm [22]).

Figure 4. Capped square prism [(Y1)O₈(F1)(F1′)]¹⁵⁻ (left), bicapped trigonal prism [(Y2)O₆F₂]¹¹⁻ (mid) and ψ¹-tetrahedron [AsO₃]³⁻ (right) with complete cationic decoration in the tetragonal crystal structure of Y₅F₃[AsO₃]₄.

The structure becomes easier to understand, if it is divided into two units, firstly the ψ¹-tetrahedron [AsO₃]³⁻ as just presented, and secondly the substructure of fluoride and yttrium. Both fluoride anions have clearly very different coordination environments. While (F1)⁻ is surrounded by five plus one Y³⁺ cations in the shape of an elongated octahedron [(F1)(Y2)₄(Y1)(Y1′)]¹⁷⁺, (F2)⁻ forms a chevron [(F2)(Y2)₂]⁵⁺ (∡(Y2–F2–Y2) = 147°) with just two Y³⁺ cations (Figure 5).

Figure 5. Capped square pyramid [(F1)(Y2)₄(Y1)(Y1′)]¹⁷⁺ (left) and chevron [(F2)(Y2)₂]⁵⁺ (right) in the crystal structure of Y₅F₃[AsO₃]₄.

The [(F1)(Y2)4(Y1)(Y1′)]¹⁷⁺ pseudo-octahedra form chains according to $\begin{array}{c}1\\ \infty \end{array}${[(F1)(Y2)$\begin{array}{c}t\\ 4/1\end{array}$(Y1)$\begin{array}{c}v\\ 2/2\end{array}$]¹⁴⁺} (t = terminal, v = vertex-connecting) through corner-linkage via Y1, which run parallel to the c-axis. These chains are connected to the [(F2)(Y2)₂]⁵⁺ chevrons via Y2 to create a three-dimensional network (Figure 6, top).

Figure 6. The yttrium-fluoride substructure linked via secondary Y⋯F’ contacts to form a three-dimensional network $\begin{array}{c}3\\ \infty \end{array}${[Y₅F₃]¹²⁺} (top) and the substructure of ψ¹-tetrahedral [AsO₃]³⁻ anions, which leave channels along [001] due to their lone pairs (bottom) in the crystal structure of Y₅F₃[AsO₃]₄ as viewed along [010] (left) and [001] (right).

Figure 7 shows the combination of both substructures ($\begin{array}{c}3\\ \infty \end{array}${[Y₅F₃]¹²⁺} + 4 [AsO₃]³⁻) with a view along [001] and the unit-cell edges marked.

Figure 7. View at the unit-cell content of Y₅F₃[AsO₃]₄ along [001] with emphasized vertex-connected [FY₅₊₁]¹⁷⁺ polyhedra and discrete ψ¹-tetrahedral [AsO₃]³⁻ units.

Table 1 summarizes the crystallographic data for Y₅F₃[AsO₃]₄, while Table 2 lists the atomic positions, Table 3 offers selected interatomic distances and Table 4 contains the motifs of mutual adjunction for its crystal structure.

Table 1. Crystallographic data of Y₅F₃[AsO₃]₄ and their determination.

Structured Formula		Y5F3[AsO3]4
Crystal system		tetragonal
Space group		P4/ncc (no. 130)
Lattice parameters,	a/pm c/pm c/a	1143.80(8) 1078.41(7) 0.943
Number of formula units, Z		4
Calculated density, Dx/g∙cm−3		4.676
Molar volume, Vm/cm3∙mol−1		212.41
Index range, ±hmax, ±kmax, ±lmax		14, 14, 13
Diffractometer limit, 2θ/°		54.9
Electron sum, F(000)/e−		1800
Absorption coefficient, μ/mm−1		29.75
Extinction coefficient, ε/pm−3		0.00106(7)
Measured reflections		20547
Independent ones		810
Rint/Rσ		0.096/0.041
Reflections with |Fo| ≥ 4σ(Fo)		620
R1/R1 with |Fo| ≥ 4σ(Fo)		0.056/0.034
wR2/GooF		0.067/1.082
Residual electron density, ρmax/min/e− 10−6 pm−3		0.80/–0.85
CSD number		2321105

Table 2. Fractional atomic coordinates, Wyckoff sites and symmetries as well as U_eq values for Y₅F₃[AsO₃]₄.

Atom	Site	x/a	y/b	z/c	Ueq/pm2
Y1	4c	1/4	1/4	0.35328(11)	134(3)
Y2	16g	0.10339(5)	0.07674(5)	0.10733(6)	130(2)
F1	4c	1/4	1/4	0.1312(7)	172(16)
F2	8f	0.0517(4)	0.9483(4)	1/4	214(13)
As	16g	0.23168(6)	0.54424(6)	0.37332(7)	147(2)
O1	16g	0.0894(4)	0.5477(4)	0.4353(4)	190(11)
O2	16g	0.2702(4)	0.4265(4)	0.4764(4)	131(10)
O3	16g	0.2223(4)	0.4409(4)	0.2465(4)	167(11)

Table 3. Selected interatomic distances (d/pm) in the crystal structure of Y₅F₃[AsO₃]₄.

[(Y1)F(1+1)O8]15− polyhedron
Y1–F1	1×	239.5(7)
Y1–O2	4×	242.8(4)
Y1–O3	4×	248.9(5)
Y1⋯F1	1×	299.7(7)
[(Y2)F2O6]11− polyhedron
Y2–F2	1×	220.8(2)
Y2–O2	1×	227.4(4)
Y2–O1	1×	227.7(5)
Y2–O3	1×	229.8(4)
Y2–O1′	1×	234.3(4)
Y2–O2′	1×	237.4(4)
Y2–O3′	1×	250.4(4)
Y2–F1	1×	260.9(1)
[AsO3]3− ψ1-tetrahedron
As–O1	1×	176.0(5)
As–O2	1×	180.1(4)
As–O3	1×	181.1(4)
[(F1)Y5+1]17+ capped pyramid
F1–Y1	1×	239.5(7)
F1–Y2	4×	260.9(1)
F1–Y1′	1×	299.7(7)
[(F2)Y2]5+ chevron
F2–Y2	2×	220.8(2)

Table 4. Motifs of mutual adjunction for the Y₅F₃[AsO₃]₄ structure.

	O1	O2	O3	F1	F2	C.N.
Y1	0/0	4/1	4/1	1 + 1/1 + 1	0/0	9 + 1
Y2	2/2	2/2	2/2	1/4	1/2	8
As	1/1	1/1	1/1	0/0	0/0	3
C.N.	3	4	4	5 + 1	2

3.2. Crystal-Structure Description of Y₅Cl₃[AsO₃]₄

In spite of their identical crystallographic prerequisites (crystal system: monoclinic, space group: C2/c, Pearson symbol: mC96, Wyckoff sequence: f¹¹e¹c¹), the crystal structure of Y₅Cl₃[AsO₃]₄ differs considerably from the one of La₅Cl₃[AsO₃]₄ [23]. This is clearly reflected by their lattice parameters (Y₅Cl₃[AsO₃]₄: a = 1860.56(9) pm, b = 536.27(3) pm, c = 1639.04(8) pm, β = 105.739(3)° versus La₅Cl₃[AsO₃]₄: a = 1803.65(9) pm, b = 548.39(3) pm, c = 1723.16(9) pm, β = 107.432(3)° [23]) already, displaying a larger a-axis and a smaller c-axis, while the b-axis and the β-angle develop as expected by replacing the bigger La³⁺ (r_i = 116 pm) with the smaller Y³⁺ cation (r_i = 102 pm). The molar volumes of 237.0 cm³∙mol⁻¹ for the yttrium juxtaposed to 244.8 cm³∙mol⁻¹ for the lanthanum compound establish the anticipated trend, however.

Common structural features are three crystallographically different RE³⁺ cations (RE = Y and La), two of them decorated with only eight oxygen atoms [(RE1,2)O₈]¹³⁻, the third one with four oxygen atoms and four chloride anions [(RE3)O₄Cl₄]⁹⁻, providing square hemi- or antiprismatical coordination spheres (Figure 8, top). All independent oxygen atoms (O1–O6) belong as ligands to As³⁺ cations (As1 and As2) to constitute two different, but very similar ψ¹-tetrahedral [AsO₃]³⁻ anions (Figure 9) with pronounced stereochemical lone-pair activity.

Figure 8. Square [(Y1,2)O₈]¹³⁻ antiprism or prism and bicapped trigonal [(Y3)O₄Cl₂Cl’₂]⁹⁻ prism in the crystal structure of Y₅Cl₃[AsO₃]₄ with their complete As³⁺ decoration (top) and their linkage via common oxygen edges to $\begin{array}{c}2\\ \infty \end{array}${[(Y1,2)O$\begin{array}{c}e\\ 8/2\end{array}$]⁵⁻} layers parallel to the (100) plane (bottom left) and $\begin{array}{c}2\\ \infty \end{array}${[(Y3)O$\begin{array}{c}t\\ 4/1\end{array}$(Cl2)$\begin{array}{c}e\\ 2/2\end{array}$(Cl1)$\begin{array}{c}v\\ 2/4\end{array}$]^6.5−} strands propagating along [010] (bottom right).

Figure 9. The two discrete ψ¹-tetrahedral [AsO₃]³⁻ anions in the crystal structure of Y₅Cl₃[AsO₃]₄ with their complete Y³⁺ decoration.

Remarkably different is the function of the two independent Cl⁻ anions, however. While Cl2 has two short contacts to Y3, Cl1 shows a peculiar coordination sphere of eight almost equidistant trivalent cations (four Y³⁺ and four As³⁺) arranged as an idealized cube (Figure 10). The other way around, (Y3)³⁺ carries two close (Cl2)⁻ and two far away (Cl1)⁻ anions (Figure 8, right) as chloride ligands.

Figure 10. Square [(Cl1)As₄Y₄]²³⁺ (left) and trigonal [(Cl2)As₄Y₂]¹⁷⁺ prism (right) in the crystal structure of Y₅Cl₃[AsO₃]₄.

Condensation of the [(Y1)O₈]¹³⁻ and [(Y2)O₈]¹³⁻ polyhedra via four common edges each leads to fluorite-like $\begin{array}{c}2\\ \infty \end{array}${[(Y1,2)O$\begin{array}{c}e\\ 8/2\end{array}$]⁵⁻} (e = edge-connecting) layers spreading out parallel to the (100) plane (Figure 8, bottom left). Their three-dimensional linkage occurs via (Y3)³⁺ cations with their Cl⁻ ligands as $\begin{array}{c}2\\ \infty \end{array}${[(Y3)O$\begin{array}{c}t\\ 4/1\end{array}$(Cl2)$\begin{array}{c}e\\ 2/2\end{array}$(Cl1)$\begin{array}{c}v\\ 2/4\end{array}$]^6.5−} (t = terminal, v = vertex-connecting) strands along [010] on the one hand (Figure 8, bottom right) and As³⁺ cations with their lone-pairs of electrons on the other (Figure 11).

Figure 11. View at the unit-cell content of monoclinic Y₅Cl₃[AsO₃]₄ along [010] with emphasized [(Y1,2)O₈]¹³⁻ and [(Y3)O₄Cl₂Cl’₂]⁹⁻ polyhedra and discrete ψ¹-tetrahedral [AsO₃]³⁻ units.

Table 5 summarizes the crystallographic data for Y₅Cl₃[AsO₃]₄, while Table 6 lists the atomic positions, Table 7 offers selected interatomic distances and Table 8 contains the motifs of mutual adjunction for its crystal structure.

Table 5. Crystallographic data of Y₅Cl₃[AsO₃]₄ and their determination.

Structured Formula		Y5Cl3[AsO3]4
Crystal system		monoclinic
Space group		C2/c (no. 15)
Lattice parameters,	a/pm b/pm c/pm β/°	1860.56(9) 536.27(3) 1639.04(8) 105.739(3)
Number of formula units, Z		4
Calculated density, Dx/g∙cm−3		4.399
Molar volume, Vm/cm3∙mol−1		237.00
Index range, ±hmax, ±kmax, ±lmax		24, 6, 20
Diffractometer limit, 2θ/°		55.0
Electron sum, F(000)/e−		1896
Absorption coefficient, μ/mm−1		27.14
Extinction coefficient, ε/pm−3		0.00026(6)
Measured reflections		19174
Independent ones		1775
Rint/Rσ		0.115/0.077
Reflections with |Fo| ≥ 4σ(Fo)		1150
R1/R1 with |Fo| ≥ 4σ(Fo)		0.095/0.054
wR2/GooF		0.131/1.048
Residual electron density, ρmax/min/e− 10−6 pm−3		1.83/–1.74
CSD number		2401390

Table 6. Fractional atomic coordinates, Wyckoff sites and symmetries as well as U_eq values for Y₅Cl₃[AsO₃]₄.

Atom	Site	x/a	y/b	z/c	Ueq/pm2
Y1	4e	0	0.1972(3)	1/4	158(4)
Y2	8f	0.49908(6)	0.2299(2)	0.41353(6)	168(3)
Y3	8f	0.14507(6)	0.2441(2)	0.45963(6)	198(3)
Cl1	4c	1/4	1/4	0	708(19)
Cl2	8f	0.25588(19)	0.2504(7)	0.3825(2)	402(8)
As1	8f	0.11822(6)	0.2590(2)	0.10200(7)	164(3)
O1	8f	0.0418(5)	0.3322(14)	0.1396(5)	268(20)
O2	8f	0.0772(4)	0.0079(13)	0.0325(4)	185(18)
O3	8f	0.0839(4)	0.4931(13)	0.0185(4)	173(18)
As2	8f	0.36980(6)	0.2556(2)	0.19349(7)	161(3)
O4	8f	0.4326(5)	0.3543(14)	0.2877(5)	226(19)
O5	8f	0.0844(5)	0.4989(13)	0.3399(4)	186(18)
O6	8f	0.4137(4)	0.4718(12)	0.1340(4)	165(18)

Table 7. Selected interatomic distances (d/pm) in the crystal structure of Y₅Cl₃[AsO₃]₄.

[(Y1)O8]13− polyhedron
Y1–O1	2×	227.4(7)
Y1–O4	2×	239.9(8)
Y1–O5	2×	244.9(7)
Y1–O6	2×	244.9(7)
[(Y2)O8]13− polyhedron
Y2–O4	1×	220.1(7)
Y2–O3	1×	232.6(7)
Y2–O1	1×	235.1(7)
Y2–O6	1×	236.8(7)
Y2–O2	1×	238.8(7)
Y2–O3′	1×	248.4(8)
Y2–O2′	1×	252.1(7)
Y2–O5	1×	255.9(8)
[(Y3)O4Cl2Cl’2]9− polyhedron
Y3–O6	1×	218.4(7)
Y3–O3	1×	219.3(7)
Y3–O2	1×	238.0(8)
Y3–O5	1×	240.7(7)
Y3–Cl2	1×	269.5(4)
Y3–Cl2‘	1×	273.9(3)
Y3⋯Cl1	1×	325.1(1)
Y3⋯Cl1‘	1×	330.3(1)
[(As1)O3]3− ψ1-tetrahedron
As1–O1	1×	174.0(8)
As1–O2	1×	179.5(8)
As1–O3	1×	183.9(7)
[(As2)O3]3− ψ1-tetrahedron
As2–O4	1×	174.7(7)
As2–O5	1×	178.2(7)
As2–O6	1×	184.3(7)
[(Cl1)Y4As4]23+ polyhedron
Cl1⋯Y3	2×	325.1(1)
Cl1⋯Y3′	2×	330.3(1)
Cl1⋯As1	2×	332.2(1)
Cl1⋯As2	2×	334.5(1)
[(Cl2)Y2As4]17+ polyhedron
Cl2–Y3	1×	269.5(4)
Cl2–Y3‘	1×	273.9(3)
Cl2⋯As1	1×	348.9(4)
Cl2⋯As1‘	1×	355.9(4)
Cl2⋯As2	1×	353.3(4)
Cl2⋯As2‘	1×	357.5(4)

Table 8. Motifs of mutual adjunction for the Y₅Cl₃[AsO₃]₄ structure.

	O1	O2	O3	O4	O5	O6	Cl1	Cl2	C.N.
Y1	2/1	0/0	0/0	2/1	2/1	2/1	0/0	0/0	8
Y2	1/1	2/2	2/2	1/1	1/1	1/1	0/0	0/0	8
Y3	0/0	1/1	1/1	0/0	1/1	1/1	0 + 2/0 + 4	2/2	6 + 2
As1	1/1	1/1	1/1	0/0	0/0	0/0	0/0	0/0	3
As2	0/0	0/0	0/0	1/1	1/1	1/1	0/0	0/0	3
C.N.	3	4	4	3	4	4	0 + 4	2

3.3. Luminescence of Eu³⁺- and Tb³⁺-Doped Samples

None of the samples exhibits a luminescence under a UV lamp of 254 and 366 nm visible by the naked eye. This is presumably due to the low doping degree of about 0.6%. The effect of the low doping concentration can especially be seen in the spectra of Y₅X₃[AsO₃]₄:Tb³⁺ (Figure 12b,d). Besides the normally recorded ⁵D₄ → ⁷F_J (J = 3–6) transitions in the range from 480 to 630 nm, also the transitions ⁵D₃ → ⁷F_J (J = 3–6) between 370 and 470 nm can be observed, adding a blue hue to the otherwise green emission. These energetically higher transitions are more intense the further away the Tb³⁺ cations are from adjacent ones, since the cross relaxation ⁵D₃ → ⁵D₄ with simultaneous excitation of a neighbouring Tb³⁺ particle from ⁷F₆ to excited ⁷F_J levels is less probable then [24]. A certain difference can be observed for the terbium-doped influence into the yttrium fluoride and chloride hosts. For X = Cl the transitions from the ⁵D₃ state are significantly stronger than for the samples with X = F, which leads to the assumption that the doped cations are somewhat closer in the latter case.

Figure 12. The luminescence spectra of (a) Y₅Cl₃[AsO₃]₄:Eu³⁺, (b) Y₅Cl₃[AsO₃]₄:Tb³⁺, (c) Y₅F₃[AsO₃]₄:Eu³⁺ and (d) Y₅F₃[AsO₃]₄:Tb³⁺. The emission spectra are displayed in red and those for excitation in black.

The excitation spectra of the terbium-doped compounds display very weak f–f transitions from the ⁷F₆ ground state between 300 and 380 nm and an intense broad band at about 235 and 260 nm originating from f–d transitions. The energetic position of these transitions hint at a rather weak coordination of the surrounding halide and oxide anions [25]. The exact position of all transitions and those of a suitable doped reference are listed in Table 9 and Table 10.

Table 9. The excitation wavelengths and the assigned transitions of Y₅X₃[AsO₃]₄:Tb³⁺ (X = Cl and F) and Tb₂[B₂(SO₄)₆] as reference from [26].

	Y5Cl3[AsO3]4:Tb3+	Y5F3[AsO3]4:Tb3+	Tb2[B2(SO4)6]
Transition exc.	Wavelength/nm	Wavelength/nm	Wavelength/nm
f–d	233, 258	237, 262	212, 253
7F6 → 5H6	–	304	301
7F6 → 5H7	–	317	316
7F6 → 5L7, 5L8	–	341	339
7F6 → 5G5	–	351	356
7F6 → 5L10	–	371	366
7F6 → 5G6	–	377	373

Table 10. The emission wavelengths and the assigned transitions of Y₅X₃[AsO₃]₄:Tb (X = Cl and F) and Y_1.998Tb_0.002[B₂(SO₄)₆] as reference from [26].

	Y5Cl3[AsO3]4:Tb3+	Y5F3[AsO3]4:Tb3+	Y2[B2(SO4)6]:Tb3+
Transition em.	Wavelength/nm	Wavelength/nm	Wavelength/nm
5D3 → 7F6	378, 382, 385	382	–
5D3 → 7F5	415, 420	417	411
5D3 → 7F4	436, 439	437, 443	434
5D3 → 7F3	456, 458, 460	–	–
5D4 → 7F6	482, 488	491	490
5D4 → 7F5	543, 547, 557	542, 547	541
5D4 → 7F4	586, 595	585, 594, 598	583, 588
5D4 → 7F3	621, 627	623	617, 621

The spectra of the europium-doped compounds (Figure 12a,c) exhibit similar features as the previously discussed terbium spectra. The transitions from the energetically higher ⁵D₁ state to ⁷F_J (J = 1, 2) in the regime from 530 to 560 nm are weakly present as well as the ⁵D₀ → ⁷F_J (J = 1–4) transitions between 580 and 710 nm. The hypersensitive ⁵D₀ → ⁷F₂ electric dipolar transition is stronger than the ⁵D₀ → ⁷F₁ magnetic dipolar transition, which is caused by the absence of inversion symmetry at the doped sites. This is in accordance with the site symmetry of the yttrium atoms’ Wyckoff positions (Table 2 and Table 6).

The excitation spectra display f–f transitions from the ⁷F₀ ground state between 350 and 530 nm. The excitation at 535 nm is due to the ⁷F₁ → ⁵D₁ transition. Additionally, a broad charge-transfer band (CT) of europium around 260 nm occurs. For X = Cl this charge transfer is split up into three peaks, possibly because of the varied surrounding at the different yttrium sites. For [EuCl₆]³⁻ in acetonitril the charge-transfer band is reported at 301 nm and for [Eu(SO₄)]⁺ at 240 nm [27], which falls into the same range as the measured ones. A compilation of the exact positions for all transitions and these of a reference can be found in Table 11 and Table 12.

Table 11. The excitation wavelengths and the assigned transitions of Y₅X₃[AsO₃]₄:Eu³⁺ (X = Cl and F) and Eu₂[B₂(SO₄)₆] as reference from [26].

	Y5Cl3[AsO3]4:Eu3+	Y5F3[AsO3]4:Eu3+	Eu2[B2(SO4)6]
Transition exc.	Wavelength/nm	Wavelength/nm	Wavelength/nm
CT	243, 278	258	265
7F0 → 5H4–7	–	319	316
7F0 → 5D4	361	362	360
7F0 → 5G3–6	–	376	374
7F0 → 5G2	–	384	381
7F0 → 5L6	393	394	392
7F1 → 5D3	–	415	412
7F0 → 5D2	464	465	–
7F0 → 5D1	526	526	–
7F1 → 5D1	535	536	–

Table 12. The emission wavelengths and the assigned transitions of Y₅X₃[AsO₃]₄:Eu³⁺ (X = Cl and F) and Eu₂[B₂(SO₄)₆] as reference from [26].

	Y5Cl3[AsO3]4:Eu3+	Y5F3[AsO3]4:Eu3+	Eu2[B2(SO4)6]
Transition em.	Wavelength/nm	Wavelength/nm	Wavelength/nm
5D1 → 7F0	–	510	–
5D1 → 7F1	–	538	–
5D1 → 7F2	–	554, 558	–
5D0 → 7F1	587	590, 593	585, 591, 595
5D0 → 7F2	613, 618	614, 619, 623	610, 615, 621
5D0 → 7F3	–	651, 654	647, 650, 653
5D0 → 7F4	703	691, 701, 706	690, 697

In contrast to the antimony compounds YSb₂O₄X (X = Cl and Br) [14], where the lone pair at the Sb³⁺ cations contributes to the relevant energy-transfer processes, no such participation of the lone pair at the As³⁺ cations could be detected for the here investigated Y₅X₃[AsO₃]₄:Eu³⁺ or Y₅X₃[AsO₃]₄:Tb³⁺ samples with X = F and Cl.

3.4. Conclusions and Outlook

With Y₅F₃[AsO₃]₄ and Y₅Cl₃[AsO₄]₄, two new compounds were obtained that have absolutely the same composition apart from the halogen, but very different crystal structures. It would be very interesting to find out, whether the structures with the heavy halides bromide (Y₅Br₃[AsO₃]₄) and iodide (Y₅I₃[AsO₃]₄) are also different or if they follow the new structure type of the chloride Y₅Cl₃[AsO₃]₄ or the lanthanoid(III) bromide derivatives Ln₅Br₃[AsO₃]₄ [28,29]. Moreover, luminescence measurements were carried out for undoped samples and those doped with Eu³⁺ and Tb³⁺ for both Y₅F₃[AsO₃]₄ and Y₅Cl₃[AsO₃]₄.