Synthesis, crystal structures and spectroscopic properties of pure YSb₂O₄Br and YSb₂O₄Cl as well as Eu³⁺- and Tb³⁺-doped samples

Abstract

The quaternary halide-containing yttrium(III) oxidoantimonates(III) YSb₂O₄Cl and YSb₂O₄Br were synthesised through solid-state reactions from the binary components (Y₂O₃, Sb₂O₃ and YX₃, X = Cl and Br) at 750 °C in evacuated fused silica ampoules with eutectic mixtures of NaX and CsX (X = Cl and Br) as fluxing agents. YSb₂O₄Cl crystallizes tetragonally in the non-centrosymmetric space group P42₁2 with unit-cell parameters of a = 773.56(4) pm and c = 878.91(6) pm, whereas YSb₂O₄Br is monoclinic (space group: P2₁/c) with a = 896.54(6) pm, b = 780.23(5) pm, c = 779.61(5) pm and β = 91.398(3)°, both for Z = 4. The two new YSb₂O₄X compounds contain [YO₈]¹³⁻ polyhedra, which are connected via four common edges to form layers (d(Y³⁺–O²⁻) = 225–254 pm) without any Y³⁺⋯X⁻ bonds (d(Y³⁺⋯X⁻) > 400 pm). Moreover, all oxygen atoms belong to ψ¹-tetrahedral [SbO₃]³⁻ units, which are either connected to four-membered rings [Sb₄O₈]⁴⁻ in the chloride (Y₂[Sb₄O₈]Cl₂ for Z = 2) or endless chains in the bromide (Y_1/2(SbO₂)Br_1/2 for Z = 8) by common vertices. With distances of 307 pm in YSb₂O₄Cl and 326 pm in YSb₂O₄Br there are not even substantial bonding Sb³⁺⋯X⁻ (X = Cl and Br) interactions at work. Luminescence spectroscopy on samples doped with trivalent europium and terbium showed an energy transfer from the oxidoantimonate(III) moieties as the sensitizer in the host structure onto the lanthanoid activators.

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Introduction

In recent years, great attention has been paid to the structural diversity of rare-earth metal(III) oxidoarsenate(III) halides owing to the beneficial inorganic lone-pair antenna at the As³⁺ cations for luminescence applications.¹ Several representatives are known with the formula RE₅X₃[AsO₃]₄ (RE = La–Lu, X = F–Br), but all their different crystal structures have the motif of isolated ψ¹-tetrahedral [AsO₃]³⁻ anions in common. The fluoride derivatives (RE = Y, Ho, Tm–Lu)^2,3 crystallize in the tetragonal space group P4/ncc with separated [AsO₃]³⁻ units that form a lone-pair channel along [001], while the chloride derivatives crystallize monoclinically (C2/c for La–Pr,^4–6 P2/c for Nd^5–7 and Sm^2,6) with a layered structure, in which the [AsO₃]³⁻ units are linked to the chloride layers via weak secondary contacts. The three known bromide derivatives (RE = Pr, Sm, Eu)² crystallize again in the monoclinic system with space group P2/c and similar coordination features as the chloride derivatives. The motif of ψ¹-tetrahedra [AsO₃]³⁻ is also present in the oxide-halide representatives RE₃OCl[AsO₃]₂ and RE₃OBr[AsO₃]₂, which crystallize tetragonally in the space groups P4₂/mnm (RE = La)^8,9 or P4₂nm (RE = Ce–Pr, Sm–Dy with X = Cl,^2,4,10 RE = Ce, Nd, Sm, Gd, Tb with X = Br²). Like in the RE₅F₃[AsO₃]₄ cases (RE = Y, Ho, Tm–Lu), all RE₃OX[AsO₃]₂ representatives have a lone-pair tunnel structure of [AsO₃]³⁻ units, but the rare-earth metal–oxygen linkage is different. Furthermore, there are RE₅O₄Cl[AsO₃]₂ members (RE = Nd, Pr),^11,12 which crystallize monoclinically in C2/m. Not only compounds with isolated [AsO₃]³⁻ anions were synthesised, but also with additional oxidoarsenate(III) units of the pyroanionic species [As₂O₅]⁴⁻ in the triclinic RE₃X₂[As₂O₅][AsO₃] examples (RE = Sm–Gd with X = Cl,^4,13 RE = Y, Ho–Yb with X = Br^3,13). These also crystallize in a layered structure (space group: P1̄), in which both the [AsO₃]³⁻ and the [As₂O₅]⁴⁻ anions are bound to the halide layers via weak secondary contacts.

The first rare-earth metal(III) oxidobismuthate(III) halide with the composition Nd_0.5Bi_2.5O₄Cl¹⁴ was synthesised by Aurivillius. In this case, the rare-earth metal cation site is mixed with bismuth(III). Only ten years later, REBi₂O₄Cl phases (RE = Y, La, Nd)¹⁵ were the first synthesised representatives without mixed occupation of the layers. Oppermann et al. extended the spectrum of these representatives at first with ErBi₂O₄I¹⁶ considerably and found all except the cerium representatives with the composition REBi₂O₄X (RE = La, Pr–Nd, Sm–Lu for X = Cl–I).^16,17 All these representatives crystallize in the tetragonal space group P4/mmm and form layered structures, in which the rare-earth metal cations are surrounded cube-shaped by eight oxygen atoms [REO₈]¹³⁻. These cubes are linked to each other via common edges. The Bi³⁺ cations form square [BiO₄]⁵⁻ (ax = axial) with four oxygen, which are linked to each other via their corners to form layers as well.

The rare-earth metal(III) oxidoantimonate(III) halides have been neglected in previous research, except for La₅F₃[SbO₃]₄,¹⁸ which crystallizes analogously to the RE₅F₃ [AsO₃]₄ series (RE = Y, Ho, Tm–Lu), and SmSb₂O₄Cl¹⁹ postulated by Oppermann et al. to be the isotypic light homologue of SmBi₂O₄Cl¹⁷ with layers of corner-linked [SbO₄]⁴⁻ polyhedra with axial lone pairs. It was not until 20 years later that the true composition could be elucidated as Sm_1.3Sb_1.7O₄Cl,^3,20 which crystallizes in principle analogously to the REBi₂O₄X family in the tetragonal space group P4/mmm. Furthermore, the analogous bromide derivative Sm_1.5Sb_1.5O₄Br^3,20 was discovered, in which there is also a mixed occupation of the antimony position of the Sb³⁺ with Sm³⁺ cations just like in Sm_1.3Sb_1.7O₄Cl. In further studies, the derivatives of the other bromides with RE = Eu–Dy^3,21–23 with the compositions RESb₂O₄Br were discovered. They crystallize in the monoclinic space group P2₁/c, but with a different linkage of the antimony–oxygen polyhedra. Here ψ¹-tetrahedral [SbO₃]³⁻ units with only three oxygen atoms are present, which are linked to each other via two corners to form chains with the Niggli formula (v = vertex-sharing, t = terminal), not showing any mixed occupation with RE³⁺ cations. Moreover, the luminescence of trivalent europium and terbium will be investigated and discussed. The oxidoantimonate(III) host structure promises to provide an energy transfer to enhance the luminescence of the 4f–4f activators. Trivalent antimony cations themselves show an efficient 5s–5p excitation that can be used as an antenna for lanthanoid activators^24–26 and antimony(III) compounds have proven to transfer energy previously.^27,28

Results and discussion

The two rare-earth metal(III) oxidoantimonate(III) halides YSb₂O₄Cl and YSb₂O₄Br were formed from Y₂O₃ and Sb₂O₃ together with YX₃ (X = Cl and Br) as colourless square platelets. YSb₂O₄Cl crystallizes in the tetragonal space group P42₁2, while YSb₂O₄Br adopts the monoclinic space group P2₁/c, just like the already known RESb₂O₄Br representatives with RE = Eu–Dy.^3,21–23 The unit-cell parameters for YSb₂O₄Cl are a = 773.56(4) pm, c = 878.91(6) pm (c/a = 1.136), while a = 896.54(6) pm, b = 780.23(5) pm, c = 779.61(5) pm and β = 91.398(3)° apply for YSb₂O₄Br. The molar volumes of the bromides decrease from EuSb₂O₄Br (V_m = 84.36 cm³ mol⁻¹) to DySb₂O₄Br (V_m = 82.57 cm³ mol⁻¹) as consequence of the lanthanoid contraction.²⁹ Despite being half as heavy the Y³⁺ cation can be classified by its ionic radius between Dy³⁺ and Ho³⁺,³⁰ which also holds here, indicated with the molar volume of 82.08 cm³ mol⁻¹ for YSb₂O₄Br. The molar volume of YSb₂O₄Cl is with 79.19 cm³ mol⁻¹ considerably smaller as compared to the bromide derivative, due to the lighter and smaller halide anion.

While the monoclinic YSb₂O₄Br shows two crystallographically distinct Y³⁺ positions, the tetragonal YSb₂O₄Cl only comprises one Y³⁺ position. In both cases, however, the Y³⁺ cations are surrounded by eight oxygen atoms that arrange themselves to square hemiprisms [YO₈]¹³⁻. The [YO₈]¹³⁻ polyhedra are linked with four of their edges via the oxygen atoms to form two-dimensional infinite layers according to the Niggli formula (e = edge-sharing, Fig. 1). These layers run parallel to the (001) plane in YSb₂O₄Cl and to the (100) plane in YSb₂O₄Br. The distances between yttrium and the oxygen atoms range between 227 and 253 pm in YSb₂O₄Cl or 225 and 253 pm in YSb₂O₄Br. These Y³⁺–O²⁻ distances are in similar intervals as in yttrium sesquioxide Y₂O₃ (d(Y³⁺–O²⁻) = 225–234 pm) with bixbyite-type structure,³¹ where Y³⁺ resides in sixfold oxygen coordination.

Fig. 1 Two-dimensional infinite layers of edge-linked square hemiprisms [YO₈]¹³⁻ in the tetragonal crystal structure of YSb₂O₄Cl (top) and in the monoclinic crystal structure of YSb₂O₄Br (bottom).

The antimony(III) cations occupy one crystallographic position in YSb₂O₄Cl, while there are two different of them in YSb₂O₄Br. Common for both structures, the Sb³⁺ cations form ψ¹-tetrahedral [SbO₃]³⁻ units with three oxygen atoms and the electron lone pair, but this is the only common feature, since in both structures they are linked differently to each other to achieve {[SbO₂]⁻} motifs. In YSb₂O₄Br, their arrangement is already known from the representatives RESb₂O₄Br (RE = Eu–Dy),^3,21–23 namely the linkage via two corners to form one-dimensional infinite chains according to the Niggli formula (v = vertex-sharing, t = terminal; Fig. 2). The bridging oxygen atoms show distances of 203–213 pm to the antimony(III) cations and are thus significantly longer than the terminal antimony–oxygen distances of 193–195 pm. Moreover, the terminal O1 atoms of (Sb1)³⁺ exhibit distances to the next (Sb2)³⁺ cation within the chain of d(Sb2⋯O1) = 317 pm (Fig. 2, red), which is approximately the same as that of the terminal O2 atom of (Sb2)³⁺ to the next (Sb1)³⁺ cation, d(Sb1⋯O2) = 316 pm, between the chains (Fig. 2, yellow). These meandering chains propagate along [001] and lie parallel within the (100) plane.

Fig. 2 One-dimensional infinite chains of corner-linked ψ¹-tetrahedra [SbO₃]³⁻ in the monoclinic crystal structure of YSb₂O₄Br, which run parallel to the [001] direction. The contacts of the terminal oxygen atoms to the next, not directly bonded Sb³⁺ cations are shown in red within the chains and in yellow between the chains.

The motif of chains occurs more frequently in crystal structures of ternary or quarternary antimony(III)–oxygen compounds. However, Sb³⁺ has than a coordination number of four and forms square ψ¹-pyramids [SbO₄]⁵⁻ edge-linked according to . Examples of representatives for this behaviour are ASbO₂ (A = K–Cs),^32,33 BaSb₂O₄Cl,³⁴ PbSbO₂Cl³⁵ and ZnSbO₂I.³⁶ Oxygen antimony chains similar as in YSb₂O₄Br can be found in LiSbO₂,³⁷ but here they are twisted to spirals and not planar. In the YSb₂O₄Br structure there are four crystallographic different oxygen atoms, whereas in YSb₂O₄Cl we have only two different ones of them. Unlike the monoclinic compounds YSb₂O₄Br and RESb₂O₄Br (RE = Gd–Dy),^3,21–23 in YSb₂O₄Cl four ψ¹-tetrahedra [SbO₃]³⁻ form a closed ring according to {[Sb₄O₈]⁴⁻} by vertex-connections (Fig. 3). These rings lie within the (001) plane. The bridging oxygen atoms have distances of 204–210 pm to the Sb³⁺ cations. In contrast, the exo-standing terminal oxygen atoms show significantly shorter contacts of 194 pm just like it is the case for the monoclinic congeners. The terminal oxygen atoms O1 have a distance of d(Sb⋯O1) = 309 pm to the next non-covalently bonded Sb³⁺ cation, which is a shorter secondary contact than in the monoclinic YSb₂O₄Br representative. Discrete units of antimony and oxygen are relatively rare, but one example would be Na₃[SbO₃],³⁸ where isolated ψ¹-tetrahedral [SbO₃]³⁻ anions (d(Sb–O) = 189 pm, 3×) are present with their full C_3v symmetry. The structural motif of separated rings is also not novel, but found in valentinite (β-Sb₂O₃).³⁹ Here they are further connected, not isolated, and show a twisted configuration. The Sb³⁺–O²⁻ bond lengths in both compounds correspond well with typical antimony–oxygen distances in both crystalline forms of Sb₂O₃ (α: senarmontite: d(Sb–O) = 198 pm,⁴⁰ β: valentinite: d(Sb–O) = 198–202 pm³⁹).

Fig. 3 Isolated rings [Sb₄O₈]⁴⁻ of four cyclically vertex-linked ψ¹-tetrahedra [SbO₃]³⁻ in the tetragonal crystal structure of YSb₂O₄Cl according to Y₂[Sb₄O₈]Cl₂. The contacts of the terminal oxygen atoms to the next, not directly bonded Sb³⁺ cations are shown in yellow.

In YSb₂O₄Br there is only one crystallographic position for the halide anion, whereas in YSb₂O₄Cl two different ones of them are present. The halide anions show a minimum distance of d(Sb⋯Cl) = 307 pm to the nearest Sb³⁺ cation in YSb₂O₄Cl and of d(Sb⋯Br) = 326 pm in YSb₂O₄Br. Their distances to the nearest Y³⁺ cation amount to d(Y⋯Cl) = 420 pm for YSb₂O₄Cl and d(Y⋯Br) = 427 pm for YSb₂O₄Br. So at these distances, one can not speak of real coordination in either structure. Between each layer of Sb³⁺ cations there is a layer of halide anions, which in the case of YSb₂O₄Br spreads out parallel to the (100) plane, but parallel to the (001) plane in the case of YSb₂O₄Cl. This halide layer has no contact or connection to any other layer, neither via X⁻⋯Sb³⁺ nor via X⁻⋯Y³⁺ bonds. However, the layer of Sb³⁺ cations enjoys linkage to the layer of Y³⁺ cations via all oxygen atoms according to in both yttrium(III) oxidoantimonate(III) halides YSb₂O₄X (X = Cl and Br).

Fig. 4 shows an extended unit cell of YSb₂O₄Br with depicted coordination spheres of the Y³⁺ and Sb³⁺ cations. The same applies to Fig. 5, which shows the extended unit cell of YSb₂O₄Cl.

Fig. 4 Extended tetragonal unit cell of YSb₂O₄Cl as viewed along [010].

Fig. 5 Extended monoclinic unit cell of YSb₂O₄Br as viewed along [001].

Since the yttrium cations are surrounded by oxidoantimonate layers, an energy transfer from these layers towards any cation doped on the yttrium site could be expected. This was verified via luminescence spectroscopy (Fig. 6 and 7), but apparently, the luminescence was quite different for all samples (Fig. 8).

Fig. 6 Fluorescence spectra of YSb₂O₄Cl doped with either Eu³⁺ (left) or Tb³⁺ (right). λ_exc. describes the wavelength used to record the excitation spectrum, whereas λ_em. represents the wavelength, at which the emission spectrum was recorded.

Fig. 7 Luminescence spectra of YSb₂O₄Br doped with either Eu³⁺ (left) or Tb³⁺ (right). λ_exc. describes the wavelength used to record the excitation spectrum, whereas λ_em. represents the wavelength, at which the emission spectrum was recorded.

Fig. 8 Comparison of the visible luminescence of the four samples YSb₂O₄Cl:Eu³⁺ (a), YSb₂O₄Cl:Tb³⁺ (b), YSb₂O₄Br:Eu³⁺ (c), and YSb₂O₄Br:Tb³⁺ (d).

YSb₂O₄Cl:Eu³⁺ shows weak, orange-red luminescence. In the spectrum, the weak emission is represented by the low signal-to-noise-ratio. The excitation spectrum is dominated by the broad charge-transfer within the oxidoantimonate host structure peaking at 397 nm. Another band at 467 nm can be attributed to 4f–4f transitions of Eu³⁺. The emission spectrum features the main emission bands typical for trivalent europium. The band peaking at 612 nm, attributed to the emission ⁵D₀→⁷F₂, is much more intense than that for ⁵D₀→⁷F₁, normally located around 595 nm. This supports the experimentally obtained site symmetry of yttrium, as the comparably strong hypersensitive transition is a very good probe for the absence of a local inversion center.

The Tb³⁺-doped sample of YSb₂O₄Cl shows two different emissions (green and blue) that mix to give a turquoise colour impression. The broad emission in the blue regime with the maximum around 485 nm can be attributed to the emission of the host structure (³P_1,2→¹S₀ transition of the Sb³⁺ lone-pair cation), which has been observed for LaOBr:Sb³⁺ (510 nm)³ and GdSb₂O₄Br (455 nm)²¹ as well and even matches with pure antimony(III) chlorides such as Cs₂NaSbCl₆ ⁴¹ upon excitation between 255 to 280 nm. Three sharp bands assigned to the 4f–4f transitions ⁵D₄→⁷F_J (J = 3, 4, 5) were also recorded. The excitation spectrum features two 4f–4f transitions with their respective maxima at 374 and 483 nm. The charge-transfer transition of the host structure is blue-shifted compared to the Eu³⁺-doped sample and peaks at 294 nm. In both spectra apparently an energy transfer between the host structure and the lanthanoid activator happens upon excitation to enhance the luminescence, but in the Tb³⁺-doped compound this transfer is obviously incomplete causing a characteristic turquoise emission colour.

The oxidoantimonate bromides show a similar luminescence, when doped with trivalent europium or terbium, but significantly more intense (“heavy-atom effect”).¹ YSb₂O₄Br:Eu³⁺ exhibits an excitation spectrum, in which the charge-transfer band of the oxidoantimonate host structure is even more dominating compared to any 4f–4f transition of Eu³⁺.

It is blue-shifted about 50 nm compared to the chloride. While the same bands were observed as in the oxidoantimonate chloride, their relative intensities are decidedly stronger.

The excitation band around 393 nm, normally the most prominent one, can be only seen as a slight shoulder. In the emission spectrum, the band attributed to the hypersensitive transition ⁵D₀→⁷F₂ is once again noticeably more intense as compared to the band of the ⁵D₀→⁷F₁ transition, since the yttrium cation occupies a site without inversion symmetry.

The emission spectrum of YSb₂O₄Br:Tb³⁺ consists of a very broad band, which we assign to the charge-transfer transition of the host structure. The sharp band of the ⁵D₄→⁷F₆ transition with a maximum around 540 nm is typically the most intense band in Tb³⁺ spectra, the other bands are not observed, due to restrictions on the recorded wavelength regime. Like in the case above, the excitation is blue-shifted and therefore not in the accessible region of the spectrum. Interestingly, the spectrum does not feature either the 4f–5d excitation or any of the typical 4f–4f transitions of Tb³⁺. This indicates that the Tb³⁺ cations are almost exclusively excited via the energy transfer from the oxidoantimonate(III) system (¹S₀→³P_1,2 transition of the Sb³⁺ lone-pair cation and/or O²⁻→Sb³⁺ charge-transfer excitation).

Table 1 lists the most important crystallographic data for YSb₂O₄Br and YSb₂O₄Cl, while Table 2 gives the atomic coordinates, Wyckoff positions and equivalent isotropic displacement parameters. Table 3 contains selected bond lengths and interatomic distances for YSb₂O₄Cl and YSb₂O₄Br.

Table 1 Crystallographic data for YSb₂O₄Cl and YSb₂O₄Br as well as their determination

		YSb2O4Cl	YSb2O4Br
a This value also represents the Flack-x parameter for non-centrosymmetric crystal structures, from which it is transferred into BASF after the TWIN refinement^41–43 as inversion twin.
Crystal system		Tetragonal	Monoclinic
Space group		P4212 (no. 90)	P21/c (no. 14)
Lattice constants	a/pm	773.56(4)	896.54(6)
	b/pm	=a/pm	780.23(5)
	c/pm	878.91(6)	779.61(5)
	β/°	90	91.398(3)
Formula units, Z		4	4
X-ray density, Dx/g cm^−3		5.454	5.803
Molar volume, Vm/cm^3 mol^−1		79.185	82.083
Diffractometer		κ-CCD (Bruker-Nonius)
Wavelength		λ = 71.07 pm (Mo-Kα)
F(000)		760	832
Tmax/°		27.48	27.39
hkl range (±hmax, ±kmax, ±lmax)		10, 10, 11	11, 10, 10
Unique reflections		606	1220
Absorption coefficient, μ/mm^−1		21.56	27.64
Absorption correction		Program X-SHAPE 2.21 ^45
Rint/Rσ		0.098/0.047	0.132/0.104
R1/R1 with |Fo| ≥ 4σ(Fo)		0.049/0.040	0.118/0.060
wR2/goodness of fit (GooF)		0.085/1.090	0.129/0.986
Structure determination and refinement		Program SHELX-97 ^43,44
Extinction coefficient, ε/10^−6 pm^−3		—	0.0008(2)
ρmax/min/e^− 10^−6 pm^−3		1.74/−1.71	2.16/−1.83
Batch scale factor (BASF)^a		0.45(6)	—
CSD number		2044973	2044975

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Table 2 Atomic coordinates, Wyckoff positions and equivalent isotropic displacement parameters for tetragonal YSb₂O₄Cl (top) and monoclinic YSb₂O₄Br (bottom)

Atom	Wyckoff site	x/a	y/b	z/c	Ueq/pm^2
Y1	2a	0	0	0	120(6)
Y2	2c	0	1/2	0.0153(4)	118(6)
Sb	8g	0.24016(8)	0.20182(8)	0.28547(9)	151(2)
O1	8g	0.0629(9)	0.2473(9)	0.1351(11)	165(17)
O2	8g	0.4648(9)	0.2483(9)	0.1705(11)	147(17)
Cl1	2b	0	0	1/2	265(19)
Cl2	2c	0	1/2	0.4929(9)	250(18)
Y	4e	0.4922(2)	0.2368(2)	0.5018(2)	232(5)
Sb1	4e	0.77879(14)	0.05212(15)	0.75550(15)	261(4)
Sb2	4e	0.22116(14)	0.00914(14)	0.79184(15)	240(4)
O1	4e	0.6339(15)	0.0071(13)	0.5739(14)	299(32)
O2	4e	0.3698(14)	0.1843(14)	0.7440(14)	285(29)
O3	4e	0.6613(13)	0.0069(13)	0.9704(13)	212(27)
O4	4e	0.6624(14)	0.2102(13)	0.2595(15)	281(30)
Br	4e	0.0153(2)	0.2371(2)	0.5044(2)	329(5)

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Table 3 Selected interatomic distances (d/pm) for tetragonal YSb₂O₄Cl (left) and monoclinic YSb₂O₄Br (right)

Distance		YSb2O4Cl	Distance		YSb2O4Br
Y1–O1	(4×)	230.3(10)	Y–O2	(1×)	224.5(12)
Y1–O2	(4×)	247.2(9)	Y–O1	(1×)	226.0(11)
Y2–O1	(4×)	227.3(9)	Y–O1′	(1×)	228.4(12)
Y2–O2	(4×)	253.6(9)	Y–O2	(1×)	234.9(11)
Y1⋯Sb	(4×)	349.06(8)	Y–O4	(1×)	246.6(12)
Y2⋯Sb	(4×)	366.9(2)	Y–O3	(1×)	252.5(11)
Sb–O1	(1×)	193.7(7)	Y–O4′	(1×)	252.7(12)
Sb–O2	(1×)	204.2(7)	Y–O3′	(1×)	252.9(10)
Sb–O2′	(1×)	209.6(7)	Y⋯Sb2	(1×)	351.0(2)
Cl1⋯Y1	(2×)	439.45(3)	Y⋯Sb1	(1×)	351.3(2)
Cl2⋯Y2	(1×)	419.8(9)	Y⋯Sb1′	(1×)	363.9(2)
Cl2⋯Y2′	(1×)	459.1(9)	Y⋯Sb2′	(1×)	367.5(2)
Cl1⋯Sb	(4×)	307.31(7)	Sb1–O1	(1×)	193.0(12)
Cl2⋯Sb	(4×)	320.5(5)	Sb1–O3	(1×)	203.1(10)
Cl2⋯Sb′	(4×)	347.8(4)	Sb1–O4	(1×)	212.9(11)
			Sb2–O2	(1×)	195.1(12)
			Sb2–O4	(1×)	204.9(11)
			Sb2–O3	(1×)	211.3(11)
			Br⋯Y	(1×)	427.6(3)
			Br⋯Y′	(1×)	468.9(3)
			Br⋯Sb1	(1×)	325.8(2)
			Br⋯Sb1′	(1×)	328.1(2)
			Br⋯Sb1′′	(1×)	357.6(2)
			Br⋯Sb1′′′	(1×)	357.6(2)
			Br⋯Sb2	(1×)	319.7(2)
			Br⋯Sb2′	(1×)	337.5(2)
			Br⋯Sb2′′	(1×)	341.9(2)
			Br⋯Sb2′′′	(1×)	364.4(2)

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Experimental section

Synthesis of YSb₂O₄Cl and YSb₂O₄Br

The yttrium(III) oxoantimonate(III) halides YSb₂O₄X (X = Cl and Br) were synthesised at elevated temperatures via solid-state reactions in evacuated silica ampoules (Quarz- und Glasbläserei Müller, Berlin-Adlershof; inner diameter: 10 mm, wall thickness: 1 mm, length: 40 mm). Yttrium oxide (Y₂O₃, ChemPur: 99.9%, 43.57 mg for the chloride, 39.51 mg for the bromide derivative), antimony sesquioxide (Sb₂O₃, ChemPur: 99.9%, 168.75 mg for the chloride, 153.00 mg for the bromide derivative) as well as yttrium chloride (YCl₃, ChemPur: 99.9%, 37.68 mg) and yttrium bromide (YBr₃, Aldrich: 99.9%, 57.49 mg) were used as reactants according to eqn (1). Eutectic mixtures of sodium chloride (NaCl, Merck: 99.9%, 126 mg) and cesium chloride (CsCl, Aldrich: 99.9%, 674 mg) were used for YSb₂O₄Cl and those of sodium bromide (NaBr, Merck: 99.9%, 203 mg) and cesium bromide (CsBr, ChemPur: 99.9%, 597 mg) for YSb₂O₄Br to improve the reaction speed and the crystal growth as fluxing agents. As doping material europium sesquioxide (Eu₂O₃, ChemPur: 99.9%, 1.7 mg) or terbium chloride (TbCl₃, Aldrich: 99.9%, 1.0 mg) were used for YSb₂O₄Cl and europium sesquioxide (Eu₂O₃: ChemPur: 99.9%, 1.5 mg) or terbium bromide (TbBr₃, Aldrich: 99.9%, 1.4 mg) for YSb₂O₄Br.

The reactants were weighed into glassy silica ampoules under inert gas (argon) inside a glove box (Glovebox Systemtechnik, GS Mega E-line), sealed under dynamic vacuum and then subjected to a defined temperature program in a muffle furnace (Nabertherm, L 9/12). This was heated at a rate of 150 K h⁻¹ to 750 °C, held there for two days, cooled with 5 K h⁻¹ to 666 °C, held for another three days, cooled with 5 K h⁻¹ to 530 °C, again held for two days, then cooled with 10 K h⁻¹ to 480 °C and finally quenched to room temperature by cutting off the power to the closed furnace. The recovered product samples were washed with 500 ml demineralised water and then dried for 2 h in a drying oven at 120 °C. Under a stereomicroscope, colourless flat, square platelets were visible, clearly larger for YSb₂O₄Cl than for YSb₂O₄Br.

Single-crystal X-ray diffraction

Suitable crystals were selected from the samples for single-crystal X-ray diffraction experiments and fixed in glass capillaries (Hilgenberg, Malsfeld; outer diameter: 0.1 mm, wall thickness: 0.01 mm) with grease. The measurements were carried out with a κ-CCD four-circle diffractometer (Bruker-Nonius, Karlsruhe) at room temperature using Mo-K_α radiation. The program package Stoe X-Area 1.86 (2018) was used for data collection and integration. The crystal structures of YSb₂O₄Cl and YSb₂O₄Br were solved in the space groups P42₁2 and P2₁/c, respectively, by direct methods and refined with the SHELX-97 program package.^42–45

Luminescence spectroscopy

The luminescence spectra of all samples were measured using a Horiba Fluoromax-4 spectrometer scanning from 220 to 800 nm at room temperature. Therefore, finely ground powder samples were filled into the sample holder and subsequently placed in the sample chamber. These measurements were conducted and evaluated with the program FluorEssence.⁴⁶ All excitation spectra were corrected to consider the xenon-lamp spectrum.

Conclusions

With YSb₂O₄Br another representative of the known series RESb₂O₄Br (RE = Eu–Dy) could be synthesised and structurally characterised. Thus it also shows the structural motif of ψ¹-tetrahedral [SbO₃]³⁻ groups linked to a meandering chain of the formula via common vertices. As first tetragonal representative of the rare-earth metal(III) oxoantimonate(III) halides YSb₂O₄Cl (space group: P42₁2) was obtained. Although its structure shows many similarities to the monoclinic YSb₂O₄Br (space group: P2₁/c) representative, it exhibits a different corner-linkage of the ψ¹-tetrahedral [SbO₃]³⁻ units resulting in closed rings.

The luminescence spectra of samples doped with trivalent europium or terbium confirmed the lack of inversion symmetry around the yttrium cations in both structures, as well as an efficient energy transfer between the oxidoantimonate(III) layers and the lanthanoid(III)-activator cations.

Author contributions

The manuscript was written through contributions of all authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Dr Falk Lissner and Dr Ingo Hartenbach for the single-crystal X-ray diffraction measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra08382a

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