Three New Lanthanum Oxoantimonate(III) Halides: Synthesis and Crystal Structure of La₅Cl₃[SbO₃]₄, La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄

Abstract

It was possible to synthesize colorless single crystals of La₅Cl₃[SbO₃]₄ (block-shaped) as well as La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄ (both needle-shaped), representing three new compounds from the system of lanthanum oxoantimonate(III) halides, which have not been described in the literature before. La₅Cl₃[SbO₃]₄ crystallizes in the monoclinic space group P2/c with the lattice parameters a = 895.82(5) pm, b = 564.28(3) pm, c = 1728.19(9) pm, and β = 90.007(2)° for Z = 2. This layered compound contains isolated ψ¹-tetrahedral [SbO₃]^3– units, square hemiprisms [LaO₈]^13–, and antiprisms [LaO₄Cl₄]⁹⁻. La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄ crystallize isotypically in the orthorhombic space group Pnma with a = 3184.69(19) pm, b = 417.78(3) pm, c = 1019.85(6) pm for the bromide and a = 3215.08(19) pm, b = 419.94(3) pm, c = 1062.89(6) pm for the iodide. Instead of isolated [SbO₃]³⁻ anions, semi-tubular features $\begin{array}{c}1\\ \infty \end{array}${[Sb₁₂O₁₉]²⁻} are present, which consist mainly of [SbO₄]⁵⁻ and few [SbO₃]³⁻ units with stereochemically active electronic lone pairs at their Sb³⁺ centers. Within these so-called “double-halfpipes”, La³⁺ is surrounded by nine oxygen atoms as [LaO₉]^15– polyhedron without any contact with X⁻ anions. Single-crystal Raman measurements were performed for La₅Cl₃[SbO₃]₄ and La₂Sb₁₂O₁₉I₄, and La₅Cl₃[SbO₃]₄ was structurally compared with the isostoichiometric, but not isotypic La₅F₃[SbO₃]₄.

1. Introduction

Because different structures for compounds with the general composition RE₅X₃[AsO₃]₄ (RE = Y, La–Nd, Sm–Lu; X = F–Br) have already been described from several previous investigations into the quaternary systems RE–X–As–O, interest has now switched to exploring this composition type by moving from arsenic to antimony as the heavier homolog. The aim was to uncover possibly existing structural similarities or differences between these halide derivatives of oxoarsenates(III) and -antimonates(III). While the fluoride derivatives with the composition RE₅F₃[AsO₃]₄ (RE = Y, Ho, Tm–Lu) [1,2,3] exhibit a tetragonal crystal structure (space group: P4/ncc), the analogous chlorides (RE₅Cl₃[AsO₃]₄ with RE = La–Nd and Sm) [2,4,5] and bromides (RE₅Br₃[AsO₃]₄ with RE = Pr, Sm, Eu, and Tb) [2,3,6] occur with different monoclinic crystal structures in the space groups P2/c or C2/c. For the corresponding oxoantimonates(III) with the composition RE₅X₃[SbO₃]₄, only the lanthanum representative La₅F₃[SbO₃] [7] has been known in literature since 1988 and shows the same crystal structure as the fluoride oxoarsenates(III) RE₅F₃[AsO₃]₄ [1,2,3]. Other antimonates(III) of this composition have not yet been described in the literature. The influence of both the lone-pair of electrons at the Pn³⁺ centers (Pn = As and Sb) of the involved [PnO₃]^3– anions and the differently hard X^– anions (X = F–I) on potential luminescence properties of the RE³⁺ cations have triggered our activities for further investigations. Moreover, the existence of completely different compositions and crystal structures for the tetragonal oxobismuthate(III) halides REBi₂O₄X (RE = Y, La, Pr, Nd, Sm–Lu, X = Cl–I) [6,8,9,10,11], crystallizing in space group P4/mmm, was encouragement enough for more systematic studies. First results on the oxoantimonate(III)-halide branch have revealed that the tetragonal RESb₂O₄Cl representatives for RE = Sm and Eu [6,12] crystallize in space group P4/ncc, but for RE = Y, Gd–Lu [3,6,13,14] in space group P42₁2, whereas the RESb₂O₄Br representatives with RE = Y, Eu–Dy [3,6,13,15,16] prefer the monoclinic space group P2₁/c. All before-mentioned investigations have so far shown that for As³⁺ exclusively isolated [AsO₃]³⁻ anions are present as ψ¹-tetrahedral trigonal pyramids, while for the Sb³⁺ case the analogous units [SbO₃]^3– aspire to an extra oxygen contact for their central Pn³⁺ cation, which becomes a general feature in the oxobismuthate(III) halides, where vertex-sharing [BiO₄]⁵⁻ units dominate as ${\mathsf{\psi }}_{\mathrm{ax}}^1$-square pyramids for the heaviest Pn³⁺ congener. The influence of the halide anions X^– (X = F–I) on the actual crystal structures seems to be intriguing as well, so we started our corresponding research with the system La–X–Sb–O.

2. Materials and Methods

2.1. Product Synthesis

According to the literature, for the synthesis of La₅F₃[SbO₃]₄, the preparation of La₅Cl₃[SbO₃]₄ was carried out according to Equation (1) with the reactants lanthanum sesquioxide (La₂O₃ (656 mg, 0.336 mmol): ChemPur, 99.99%), lanthanum trichloride (LaCl₃ (246 mg, 0.336 mmol): ChemPur, 99.9%), antimony sesquioxide (Sb₂O₃ (583 mg, 0.168 mmol): ChemPur, 99.9%) and as flux cesium chloride (CsCl (800 mg): Aldrich, 99.9%) at a temperature of 780 °C for a period of four days, followed by slow cooling to 660 °C and further kee** this temperature for four more days, and subsequent cooling to room temperature. Thereafter, the aqueous workup to remove the flux yielded a fibrous microcrystalline powder containing a few octahedrally shaped single crystals (Figure 1), which had the desired composition and could be further characterized by single-crystal X-ray diffraction.

2 La₂O₃ + LaX₃ + 2 Sb₂O₃ → La₅X₃[SbO₃]₄ (Flux: CsX, X = Cl)

(1)

In synthesis experiments of LaSb₂O₄Br and LaSb₂O₄I according to reaction Equation (2), lanthanum sesquioxide (La₂O₃ (52 mg, 0.158 mmol or 47 mg, 0.145 mmol): ChemPur, 99.99%) was reacted with the lanthanum trihalides (LaBr₃ (60 mg, 0.158 mmol): ChemPur, 99.9%; LaI₃ (76 mg, 0.145 mmol): ChemPur, 99.9%, respectively) and antimony sesquioxide (Sb₂O₃ (138 mg, 0.053 mmol or 127 mg, 0.048 mmol): ChemPur, 99.9%) to yield the new lanthanum oxoantimonate(III) halides La₂Sb₁₂O₁₉X₄ (X = Br or I). Cesium bromide (CsBr (800 mg): Aldrich, 99.9%) or cesium iodide (CsI (800 mg): Merck, 99.99%) were chosen as fluxes, and the reactions always took place in evacuated glassy silica ampoules. Their content reacted in a muffle furnace (Nabertherm, L9/12) at a specific temperature program. This involved heating to 750 °C at a rate of 150 °C/h and holding this temperature for two more days. Then, cooling with a rate of 5 °C/h brought the vials to 666 °C, and again, kee** this temperature for two days was useful. Renewed cooling at 5 °C/h took the ampoules down to 530 °C, whereafter this temperature was maintained for two more days. In the final step, cooling at 10 °C/h to 480 °C and finally at 150 °C/h to room temperature took place. The reaction products were then washed with demineralized water and dried at 120 °C in a drying oven. Needle-shaped single crystals (Figure 2) were easily found under a stereomicroscope.

La₂O₃ + LaX₃ + 3 Sb₂O₃ → 3 LaSb₂O₄X (Flux: CsX, X = Br and I)

(2)

2.2. Single-Crystal X-ray Diffraction

Suitable crystals of all three lanthanum oxoantimonate(III) halides were selected from the samples and fixed in glass capillaries (Hilgenberg, Malsfeld; outer diameter: 0.1 mm, wall thickness: 0.001 mm) with grease. The measurements occurred with a four-circle single-crystal diffractometer κ-CCD (Bruker-Nonius, Karlsruhe, Germany) for La₅Cl₃[SbO₃]₄ and a single-crystal diffractometer STADI-VARI (Stoe, Darmstadt, Germany) for La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄. The monoclinic crystal structure of La₅Cl₃[SbO₃]₄ was solved using direct methods in the centrosymmetric space group P2/c and refined using the SHELX-97 program package [18,19]. The orthorhombic crystal structures of La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄ were determined in the centrosymmetric space group Pnma with the same methods.

2.3. Raman Spectroscopy

Raman spectra of the single crystals of La₅Cl₃[SbO₃]₄ and La₂Sb₁₂O₁₉I₄ were recorded using a Raman microscope (XploRA, Horiba, Kyoto, Japan) with an excitation wavelength of λ = 638 nm (La₅Cl₃[SbO₃]₄) and λ = 532 nm (La₂Sb₁₂O₁₉I₄) at a LASER power of 25 mW.

2.4. Electron-Beam Microprobe Analysis

Scanning electron microscope (SEM) images of La₅Cl₃[SbO₃]₄ and La₂Sb₁₂O₁₉Br₄ were acquired using an electron-beam microprobe system (SX-100, Cameca, Gennevilliers, France).

2.5. Powder X-ray Diffraction

The sample of La₅Cl₃[SbO₃]₄ was measured in transmission geometry on a Rigaku SmartLab X-ray powder diffractometer (Rigaku, Tokyo, Japan) using Cu-K_α1 radiation. Figure 3 shows the powder diffractogram. It contains traces of CsCl and an unknown phase (marked with 2) in addition to La₅Cl₃[SbO₃]₄. Further reflections of elemental cubic aluminum (marked with 1) result from the measurement on an aluminum sample carrier.

The analogously recorded powder X-ray diffractograms of La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄ are not shown, despite the fact that they also show the presence of the title compounds. Moreover, they document that these products do not occur in phase pure, which is already assumable by their formation reaction (Equation (2)), targeting the compositions LaSb₂O₄Br and LaSb₂O₄I. In the first case, there is strong evidence for the presence of La₅Br₃[SbO₃]₄ as a by-product, which most probably crystallizes isotypically with La₅Cl₃[SbO₃]₄. The formation of this compound with a molar ratio of La:Sb = 5:4 would explain the dominance of the La₂Sb₁₂O₁₉Br₄ phase (La:Sb ratio = 1:6) when a compound such as LaSb₂O₄Br (La:Sb ratio = 1:2) is planned to be synthesized. For the La₂Sb₁₂O₁₉I₄ case, there are many extra reflections in the powder X-ray diffractogram, which originate from a cesium-containing by-product with the composition CsI₄La₂Sb_8.333O₁₄ (monoclinic, C2/m; a ≈ 2325 pm, b ≈ 420 pm, c ≈ 1300 pm, β ≈ 96.5° for Z = 2) crystallizing isotypically with RbI₄Nd₂Sb_8.333O₁₄ [20,21] after the incorporation of some CsI from the flux (Equation (2)). As the empirical formula Cs₂I₈La₄Sb_16.667O₂₈ for Z = 1 already suggests, there are strong structural similarities to La₂Sb₁₂O₁₉I₄, which will be shown in an upcoming publication soon.

3. Results and Discussion

3.1. Crystal Structures of La₅Cl₃[SbO₃]₄ and La₅F₃[SbO₃]₄ in Comparison

La₅Cl₃[SbO₃]₄ crystallizes monoclinically in the space group P2/c with the lattice parameters a = 895.82(5) pm, b = 564.28(3) pm, c = 1728.19(9) pm and β = 90.007(2)° for two formula units per unit cell. Three distinct crystallographic positions result for the La³⁺ cations, each having a coordination number of C.N. = 8, but the coordinating particles differ to some extent. (La1)³⁺ and (La2)³⁺ are surrounded eightfold by oxygen atoms in the form of square antiprisms or hemiprisms [LaO₈]¹³⁻ with lanthanum-oxygen distances in the range of d(La1–O) = 237–264 pm and d(La2–O) = 230–262 pm (Figure 4, left and mid). These values are in good agreement with those in lanthanum sesquioxide (La₂O₃, A-type), in which interatomic distances of 237–273 pm occur for C.N. = 7 [22]. The [(La1,2)O₈]¹³⁻ polyhedra are linked by four common edges each in such a way that infinite layers parallel to the bc plane are formed, satisfying the Niggli formula $\begin{array}{c}2\\ \infty \end{array}${[LaO$\begin{array}{c}e\\ 8/2\end{array}$]⁵⁻} (Figure 5). According to the crystallographic multiplicities, each (La1)³⁺-centered polyhedron is surrounded by four (La2)³⁺-centered ones, but each (La2)³⁺-centered polyhedron is surrounded by two (La1)³⁺- and two (La2)³⁺-centered ones.

In contrast, the third La³⁺ cation also has a coordination number of eight but is coordinated by four oxygen atoms and four Cl^– anions in the center of a square antiprism [(La3)O₄Cl₄]⁹⁻ (Figure 4, right). The values of the interatomic distances are d(La3–O) = 232–245 pm and d(La3–Cl) = 323–326 pm, so the lengths of the lanthanum–oxygen bonds are more similar to those in the lanthanum oxide chloride LaOCl (PbFCl-type, 4 × 239 pm) [20]. The comparison of the distances to the four Cl^– anions also shows better agreement with the five values of 313–321 pm for LaOCl [23] compared to those in the lanthanum trichloride LaCl₃ (UCl₃-type), which exhibits considerably shorter interatomic distances of 295–296 pm for C.N. = 9 [24].

The coordination polyhedra around (La3)³⁺ show a different linkage pattern compared to those centered with (La1)³⁺ and (La2)³⁺. Namely, the linkage occurs once via the (Cl2)⁻ anions, which edge-link the polyhedra along the b-axis, forming one-dimensional double strands. A quite similar bonding situation is found in the rare-earth metal(III) halide oxoarsenates(III) RE₃X₂[As₂O₅][AsO₃] (RE = Y, Sm–Gd, Ho–Yb and X = Cl and Br) [3,25]. However, these chains are still corner-linked to each other via the (Cl1)^– anions here, turning the double strands into a two-dimensional bilayer (Figure 6, top), which can be described with the Niggli formula $\begin{array}{c}2\\ \infty \end{array}${[(La3)O$\begin{array}{c}t\\ 4/1\end{array}$(Cl1)$\begin{array}{c}v\\ 1/2\end{array}$(Cl2)$\begin{array}{c}e\\ 3/3\end{array}$]^6.5–}. The two different Cl^– anions that contribute significantly to the linkage of the [(La3)O₄Cl₄]^9– antiprisms are shown again in Figure 6 (bottom) in the centers of cationic coordination polyhedra, where the different coordination numbers of C.N.(Cl1) = 2 and C.N.(Cl2) = 3 are particularly prominent.

The crystal structure of La₅Cl₃[SbO₃]₄ exhibits two crystallographically distinct Sb³⁺ cations, both of which actuate a coordination number of C.N. = 3 with respect to oxygen atoms to form ψ¹-tetrahedral [SbO₃]^3– anions with antimony–oxygen distances of d(Sb1–O) = 196–202 pm and slightly shorter ones of d(Sb2–O) = 187–199 pm (Figure 7). Literature comparison with typical Sb³⁺–O²⁻ bond lengths reveals values of 198–202 pm for valentinite and senarmontite, the two naturally occurring crystalline modifications of antimony sesquioxide (Sb₂O₃), which are in good agreement [26,27]. With O–Sb–O angles from 83 to 105°, the deflection of the Sb³⁺ cations from the triangular oxygen plane of their trigonal-pyramidal [SbO₃]³⁻ anions amounts to 110–111 pm.

Figure 8 shows a section of the crystal structure of La₅Cl₃[SbO₃]₄, emphasizing the cell edges and the different coordination polyhedra. The crystal already showed difficulties in revealing the correct metric during the measurement since an orthorhombic cell was initially suggested due to the measured monoclinic angle of β = 90.007(2)°. However, only the solution and refinement in the monoclinic space group P2/c provided a reasonable structure model. This refinement was investigated using the software PLATON [28] and the subroutine Addsym, which searches for overlooked symmetry operations in the case of higher symmetry structures described in a lower symmetry space group and proposes a higher symmetry space group. However, this algorithm did not find a more suitable space group, even for a nonfit proportion of about 20%. However, the nearly orthogonal monoclinic angle and the refined twin proportions of about 60% for the major specimen and 40% for the minor one suggest that a higher-symmetric orthorhombic high-temperature modification might exist and that a symmetry break to the monoclinic structure occurs during the course of synthesis upon cooling.

In contrast to La₅Cl₃[SbO₃]₄, which finds its analogy in the rare-earth metal(III) oxoarsenates(III) Ln₅Br₃[AsO₃]₄ (Ln = Pr, Sm, Eu, and Tb) [2,3,6], La₅F₃[SbO₃]₄ [7] crystallizes isotypically to the rare-earth metal(III) fluoride oxoarsenates(III) RE₅F₃[AsO₃]₄ (RE = Y, Ho, Tm–Lu) [2,3] in the tetragonal space group P4/ncc with a = 1208 pm and c = 1144 pm (c/a = 0.947) for Z = 4. Although La₅F₃[SbO₃]₄ and La₅Cl₃[SbO₃]₄ share the same structured molecular formula, they thus have little in common, except for their discrete ψ¹-tetrahedral [SbO₃]^3– and two differently coordinated X^– anions (C.N.(F1) = 5 + 1, C.N.(F2) = 2), starting with the coordination spheres of the La³⁺ cations. While three La³⁺-cation positions are present in La₅Cl₃[SbO₃]₄, of which only (La3)³⁺ has direct contact with four Cl⁻ anions according to [(La3)Cl₄O₄]⁹⁻, two lanthanum sites can be found in La₅F₃[SbO₃]₄, which both have contact to F^– anions. The (La1)³⁺ cation is coordinated by eight oxygen atoms in the shape of a square hemiprism with one square face capped by the (F1)^– anion at a distance of 257 pm. Another (F1)^– anion is located above the opposite square face, but it has a distance of 314 pm, which no longer corresponds to a significant chemical bond (Figure 9, left). The (La2)³⁺ cations are surrounded by six O^2– and two F⁻ anions each, arranged as a bicapped trigonal prism (Figure 9, right). At the same time, the prism is spanned by one (F2)⁻, one (O1)²⁻, two (O2)²⁻ and two (O3)²⁻ anions, while another (O1)²⁻ and one (F1)^– anion recruit the caps. Therefore, in La₅F₃[SbO₃]₄, a three-dimensionally cross-linked network is present, whereas for La₅Cl₃[SbO₃]₄, a layered structure occurs.

The crystallographic data of La₅Cl₃[SbO₃]₄ and other parameters, such as fractional atomic coordinates and selected bond lengths, are summarized in

Table 1. Crystallographic data of La₅Cl₃[SbO₃]₄ and their determination.

Compound		La5Cl3[SbO3]4
Crystal system		monoclinic
Space group		P2/c (no. 13)
Lattice parameters,	a/pm	895.82(5)
	b/pm	564.28(3)
	c/pm	1728.19(9)
	β/°	90.007(2)
Number of formula units, Z		2
Calculated density, Dx/g∙cm−3		5.626
Molar volume, Vm/cm3∙mol−1		263.05
Diffractometer		κ-CCD (Bruker-Nonius)
Wavelength		λ = 71.07 pm (Mo-Kα)
Electron sum, F(000)/e–		1272
Measurement limit, Θmax/°		27.49
Measurement range (±hmax, ±kmax, ±lmax)		11, 7, 22
Number of measured reflections		14795
Number of symmetry-independent ones		2015
Absorption coefficient, µ/mm−1		18.52
Absorption correction		Program X-SHAPE 2.21 [18]
Rint/ Rσ		0.058/0.031
R1/R1 with |Fo| ≥ 4σ(Fo)		0.049/0.037
wR2/Goodness of Fit (GooF)		0.085/1.093
Structure determination and refinement		Program SHELX-97 [19]
Extinction coefficient, ε/10−6 pm−3		0.00075(6)
Residual electron density, ρmax/min/e− 10−6 pm−3		4.03/−3.89
Batch scale factor (BASF) 1		0.396(2)
CSD number		2214525

¹ This value represents the percentage of the twin individuum.

,

Table 2. Fractional atomic coordinates, Wyckoff sites, and equivalent isotropic displacement parameters for La₅Cl₃[SbO₃]₄.

Atom	Site	x/a	y/b	z/c	Ueq/pm2
La1	2e	0	0.2301(3)	1/4	248(3)
La2	4g	0.00064(9)	0.74327(17)	0.41548(5)	129(2)
La3	4g	0.31942(9)	0.24959(17)	0.41245(5)	149(2)
Cl1	2f	1/2	0.2476(9)	1/4	306(14)
Cl2	4g	0.4983(4)	0.7512(7)	0.4259(2)	247(8)
Sb1	4g	0.26331(11)	0.25317(18)	0.06713(5)	141(2)
Sb2	4g	0.28708(11)	0.75412(18)	0.26472(5)	148(3)
O1	4g	0.0940(16)	0.313(3)	0.1250(7)	713(51)
O2	4g	0.1748(12)	0.004(2)	0.0007(7)	217(22)
O3	4g	0.1797(12)	0.499(2)	0.4960(7)	203(22)
O4	4g	0.1406(15)	0.865(2)	0.1890(7)	481(38)
O5	4g	0.1805(13)	0.506(2)	0.3250(7)	215(27)
O6	4g	0.1858(14)	0.993(2)	0.3339(7)	270(33)

and

Table 3. Selected interatomic distances (d/pm) in La₅Cl₃[SbO₃]₄ and bond angles (∡/°) of the ψ¹-tetrahedral [SbO₃]³⁻ anions.

Bond		d/pm	Bond		d/pm
La1–O1	2×	236.5(13)	Sb1–O1	1×	184.8(14)
La1–O6	2×	258.2(12)	Sb1–O2	1×	198.2(12)
La1–O5	2×	259.1(11)	Sb1–O3	1×	200.9(12)
La1–O1	2×	263.3(13)
			Sb2–O4	1×	195.7(13)
La2–O4	1×	230.9(12)	Sb2–O5	1×	199.0(12)
La2–O3	1×	253.3(11)	Sb3–O6	1×	201.6(11)
La2–O2	1×	257.6(11)
La2–O6	1×	259.3(13)	Atom triple		∡/°
La2–O2’	1×	259.5(12)	O1–Sb1–O2	1×	96.6(8)
La2–O3’	1×	261.0(12)	O2–Sb1–O3	1×	89.5(5)
La2–O5	1×	261.5(11)	O3–Sb1–O1	1×	84.2(7)
La2–O1	1×	266.6(14)
			O4–Sb2–O5	1×	104.7(6)
La3–O6	1×	231.8(11)	O5–Sb2–O6	1×	86.8(5)
La3–O3	1×	237.2(12)	O6–Sb2–O4	1×	83.1(5)
La3–O5	1×	243.4(12)
La3–O2	1×	246.0(13)
La3–Cl2	1×	323.6(4)
La3–Cl1	1×	324.0(3)
La3–Cl2’	1×	324.5(4)
La3–Cl2″	1×	326.0(4)

.

3.2. Raman Spectroscopy on La₅Cl₃[SbO₃]₄

A single-crystal Raman measurement was performed for further characterization to prove the relationship to the rare-earth metal(III) bromide oxoarsenates(III) RE₅Br₃[AsO₃]₄ (RE = Pr, Sm, Eu, Tb) [2,3,6]. The spectrum recorded at an excitation wavelength of λ = 638 nm is shown in Figure 10.

The symmetric Sb³⁺–O²⁻ valence vibration can be identified as the second strongest band, which is located at a wavenumber of about 721 cm^–1 for La₅Cl₃[SbO₃]₄. The band found at 621 cm⁻¹ has a much lower intensity and can be assigned to asymmetric valence vibration. The other bands at 523 and 462 cm⁻¹ belong to the symmetric and at 388, 327, and 296 cm⁻¹ to the asymmetric deformation vibrations. Finally, at 212 and 145 cm⁻¹, the lattice vibrations occur. Compared to the rare-earth metal(III) bromide oxoarsenates(III) RE₅Br₃[AsO₃]₄, the spectrum is almost the same, but the peaks are much better defined and only shifted by a few wavenumbers [29,30]. As with the rare-earth metal(III) oxoantimonate(III) chlorides of composition RESb₂O₄Cl (RE = Y, Sm–Lu) [3,6,12,13,14], the common mode valence vibration of the terminal oxygen atoms to the antimony is similarly pronounced at about 700–720 cm⁻¹. However, La₅Cl₃[SbO₃]₄ lacks the push-pull vibration, which is found in the range of 660 cm⁻¹ for GdSb₂O₄Cl and at 672 cm⁻¹ for SmSb₂O₄Cl, since the latter contains discrete [Sb₄O₈]⁴⁻ rings of four cyclically vertex-connected ψ¹-tetrahedral [SbO₃]³⁻ units according to RE₂[Sb₄O₈]Cl₂ [3,6,12,13,14].

3.3. Crystal Structure of La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄

Synthesis experiments to prepare LaSb₂O₄Br and LaSb₂O₄I have failed so far but yielded two isostructural lanthanum oxoantimonate(III) halides with the composition La₂Sb₁₂O₁₉X₄ (X = Br and I). Both crystallize orthorhombically in the space group Pnma with Z = 2. The unit-cell parameters for the bromide are a = 3184.69(19) pm, b = 417.78(3) pm, c = 1019.85(6) pm, while for the iodide, they amount to a = 3215.08(19) pm, b = 419.94(3) pm, c = 1062.89(6) pm, so as expected, the iodide shows larger values as compared to the bromide owing to the larger radius of the iodide anion (r_i(I^–) = 220 pm) versus r_i(Br⁻) = 195 pm). One of the characteristic structural features of these compounds becomes apparent when considering the environment of the unique La³⁺ cation. This crystallographically singular cation is coordinated by nine oxygen atoms arranged as a capped square antiprism. These [LaO₉]¹⁵⁻ polyhedra form an endless strand along the b-axis according to $\begin{array}{c}1\\ \infty \end{array}${[LaO$\begin{array}{c}t\\ 5/1\end{array}$O$\begin{array}{c}e\\ 4/2\end{array}$]¹¹⁻}, which undergoes linkage via two polyhedral edges (2 × O4···O7). The lanthanum–oxygen distances fall into the range of d(La–O) = 239–297 pm for the bromide and d(La–O) = 240–296 pm for the iodide derivative (Figure 11). These numbers agree quite well with literature values of d(La–O) = 237–273 pm for C.N. = 7 in A-type La₂O₃ [22] and even better with PbFCl-type LaOBr (d(La–O) = 240 pm) [31] and LaOI (d(La–O) = 241 pm) [32] although there are no X^– anions in the coordination sphere of La³⁺ in the La₂Sb₁₂O₁₉X₄ cases.

Another substructure within these compounds is that of the antimony–oxygen environment. Most of the Sb³⁺ cations, together with four oxygen atoms, form trigonal ${\mathsf{\psi }}_{eq}^1$-bipyramids [SbO₄]⁵⁻ (Sb1–Sb5, Figure 12), which are linked to each other either by an edge (Sb2 with Sb3 and Sb1 with Sb4) or by a corner (Sb1 with Sb3 and Sb5 with Sb4, as well as Sb5 with Sb6). In contrast, the (Sb6)³⁺ cations form ψ¹-tetrahedra [SbO₃]³⁻ (Figure 12) with only three oxygen atoms each, which in turn are linked to one another via edges (O6···O6). The oxygen atom O7 remains terminal, and both O6 atoms link two open “halfpipes” with two (Sb6)³⁺ cations each as linkers to form a “double-halfpipe” $\begin{array}{c}1\\ \infty \end{array}${[Sb₁₂O₁₉]²⁻} (Figure 12). Within these so-called “double-halfpipes”, the antimony–oxygen distances range from 197 to 225 pm (plus 254 pm) for La₂Sb₁₂O₁₉Br₄ and from 192 to 227 pm (plus 256 pm) for La₂Sb₁₂O₁₉I₄, which agree quite well with literature values of d(Sb–O) = 198–202 pm (plus 251 and 262 pm) in valentinite (β-Sb₂O₃) [27] and come slightly higher than d(Sb–O) = 198 pm (3×) in senarmontite (α-Sb₂O₃) [26], but show a high degree of diversity.

The capped square hemiprisms [LaO₉]¹⁵⁻ are located within these “double-halfpipes”, where each oxygen atom of every [LaO₉]¹⁵⁻ polyhedron is also a component of the antimony-oxygen “double-halfpipe” arrangement (Figure 13).

Furthermore, the environment of the heavy halide anions X⁻ (X = Br and I) is also very interesting. (X1)⁻ is surrounded by ten Sb³⁺ cations with the shape of a bicapped square prism, while (X2)^– has nine Sb³⁺ cations arranged as monocapped square prisms as nearest neighbors. Therefore, once again, no bonding La³⁺···X⁻ contacts occur since the shortest La³⁺···X⁻ distances are 332 pm for X = Br and 342 pm for X = I. There are always two of these prisms [(X1)Sb₁₀]²⁹⁺ and [(X2)Sb₉]²⁶⁺ linked together to form a double unit via a shared face (2× Sb2 + 2× Sb4). Figure 14 also shows that the (Sb6)³⁺ cations can be present only in half of their abundance in both cases to avoid too short distances between two of them (110 pm for X = Br, 104 pm for X = I) at the corresponding corners of each [(X2)Sb₉]²⁶⁺ polyhedron. The antimony–halide distances range from 332 to 375 pm for La₂Sb₁₂O₁₉Br₄ and from 342 to 385 pm for La₂Sb₁₂O₁₉I₄, so one can not speak of real chemical bonds here, but only of long secondary contacts.

The content of an extended unit cell of the crystal structure of both La₂Sb₁₂O₁₉X₄ representatives (X = Br and I) as viewed along [010] can be seen in Figure 15, which suggests the impression of a hexagonal packing of rods or better “double-halfpipes” $\begin{array}{c}1\\ \infty \end{array}${[Sb₁₂O₁₉]²⁻}, encapsulating the La³⁺ cations to form one $\begin{array}{c}1\\ \infty \end{array}${[LaO$\begin{array}{c}t\\ 5/1\end{array}$O$\begin{array}{c}e\\ 4/2\end{array}$]¹¹⁻} strand per “halfpipe”.

Tubular or semi-tubular arrangements of antimony and oxygen are not so rare, especially when heavy anions (X^– = Br⁻ and I⁻) occur in antimony(III) oxide halides, such as Sb₈O₁₁X₂ (X = Cl–I) [33,34,35], Sb₅O₇I [36,37,38], and Sb₃O₄I [39]. Even with additional alkali-metal cations, they persist, for example, in the systems AI₄RE₂Sb_8.333O₁₄ (A = K–Cs; RE = Y, Pr–Tm) [6,20,21].

The crystallographic data of La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄ and other parameters, such as fractional atomic coordinates and selected bond lengths, are summarized in

Table 4. Crystallographic data of La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄ and their determination.

Compound		La2Sb12O19Br4	La2Sb12O19I4
Crystal system		orthorhombic
Space group		Pnma (no. 62)
Lattice parameters,	a/pm	3184.69(19)	3215.08(19)
	b/pm	417.78(3)	419.94(3)
	c/pm	1019.85(6)	1062.89(6)
Number of formula units, Z		2
Calculated density, Dx/g∙cm−3		5.782	5.902
Molar volume, Vm/cm3∙mol−1		408.60	432.15
Diffractometer		STADI-VARI (Stoe & Cie)	κ-CCD (Bruker-Nonius)
Wavelength		λ = 56.08 pm (Ag-Kα)	λ = 71.07 pm (Mo-Kα)
Electron sum, F(000)/e−		2036	2180
Measurement limit, Θmax/°		30.66	27.48
Measurement range (±hmax, ±kmax, ±lmax)		57, 7, 18	41, 5, 13
Number of measured reflections		33659	22948
Number of symmetry-independent ones		4554	1873
Absorption coefficient, µ/mm−1		11.01	18.37
Absorption correction		Program X-SHAPE 2.21 [18]
Rint/ Rσ		0.069/0.059	0.175/0.092
R1/R1 with |Fo| ≥ 4σ(Fo)		0.104/0.066	0.119/0.063
wR2/Goodness of Fit (GooF)		0.173/1.043	0.161/0.985
Structure determination and refinement		Program SHELX-97 [19]
Residual electron density, ρmax/min/e− 10−6 pm−3		7.77/−7.31	5.50/−3.18
CSD number		2250889	2250901

,

Table 5. Fractional atomic coordinates, Wyckoff sites, and equivalent isotropic displacement parameters of La₂Sb₁₂O₁₉Br₄.

Atom	Site	s.o.f.	x/a	y/b	z/c	Ueq/pm2
La	4c	1	0.40012(2)	¼	0.64836(6)	147(1)
Sb1	4c	1	0.29137(2)	¼	0.81595(8)	183(2)
Sb2	4c	1	0.13081(2)	¼	0.86234(7)	130(1)
Sb3	4c	1	0.29463(2)	¼	0.46716(7)	127(1)
Sb4	4c	1	0.13980(3)	¼	0.47176(8)	279(2)
Sb5	4c	1	0.03442(3)	¼	0.38192(9)	332(2)
Sb6	8d	0.500(3) (a)	0.01684(3)	0.1180(4)	0.87741(12)	269(4)
O1	4c	1	0.1884(2)	¼	0.9352(8)	146(12)
O2	4c	1	0.3206(3)	¼	0.6449(8)	186(14)
O3	4c	1	0.3562(2)	¼	0.4099(8)	153(13)
O4	4c	1	0.0953(3)	¼	0.3314(10)	240(17)
O5	4c	1	0.1887(3)	¼	0.3465(10)	262(18)
O6	4c	1	0.4700(3)	¼	0.5066(9)	225(16)
O7	4c	1	0.1027(2)	¼	0.0347(8)	173(14)
O8	4c	1	0.3506(3)	¼	0.8952(11)	309(20)
O9	4c	0.73(4) (b)	0.4643(5)	¼	0.8020(16)	383(52)
O10	4c	0.77(4) (b)	0.0204(5)	¼	0.1941(16)	442(49)
Br1	4c	1	0.27927(5)	¼	0.14307(13)	318(3)
Br2	4c	1	0.43006(4)	¼	0.12576(14)	302(3)

^(a) freely refined fractional site occupation factor (s.o.f.); ^(b) constraintly refined site occupation factors with s.o.f.(O9) + s.o.f.(O10) = 1.5.

,

Table 6. Fractional atomic coordinates, Wyckoff sites, and equivalent isotropic displacement parameters of La₂Sb₁₂O₁₉I₄.

Atom	Site	s.o.f.	x/a	y/b	z/c	Ueq/pm2
La	4c	1	0.39978(5)	¼	0.64015(13)	165(4)
Sb1	4c	1	0.29170(5)	¼	0.80393(14)	186(4)
Sb2	4c	1	0.13171(5)	¼	0.86652(14)	145(4)
Sb3	4c	1	0.29476(5)	¼	0.46760(14)	132(4)
Sb4	4c	1	0.13978(6)	¼	0.45113(15)	279(5)
Sb5	4c	1	0.03568(6)	¼	0.36641(16)	370(6)
Sb6	8d	0.500(4) (a)	0.01618(7)	0.1258(6)	0.88214(19)	275(8)
O1	4c	1	0.1888(5)	¼	0.9406(14)	170(40)
O2	4c	1	0.3194(6)	¼	0.6373(14)	278(45)
O3	4c	1	0.3563(5)	¼	0.4150(14)	188(38)
O4	4c	1	0.0953(5)	¼	0.3193(16)	288(46)
O5	4c	1	0.1897(6)	¼	0.3309(17)	361(49)
O6	4c	1	0.4694(5)	¼	0.5037(16)	257(42)
O7	4c	1	0.1029(5)	¼	0.0307(15)	223(41)
O8	4c	1	0.3505(6)	¼	0.8757(18)	489(60)
O9	4c	0.74(5) (b)	0.4614(8)	¼	0.786(3)	916(167)
O10	4c	0.76(5) (b)	0.0214(8)	¼	0.191(2)	788(142)
I1	4c	1	0.28347(5)	¼	0.13865(14)	224(4)
I2	4c	1	0.42943(6)	¼	0.12643(15)	294(5)

^(a) freely refined fractional site occupation factor (s.o.f.); ^(b) constraintly refined site occupation factors with s.o.f.(O9) + s.o.f.(O10) = 1.5.

and

Table 7. Selected interatomic distances (d/pm) in La₂Sb₁₂O₁₉Br₄ (left) and La₂Sb₁₂O₁₉I₄ (right).

Bond		d/pm	Bond		d/pm
La–O2	1×	253.3(9)	La–O2	1×	258.5(19)
La–O3	1×	280.6(8)	La–O3	1×	277.2(15)
La–O4	2×	280.5(7)	La–O4	2×	283.9(12)
La–O6	1×	265.3(9)	La–O6	1×	266.8(16)
La–O7	2×	239.1(4)	La–O7	2×	240.2(8)
La–O8	1×	297.1(11)	La–O8	1×	296.4(19)
La–O9	1×	257.5(16)	La–O9	1×	252(2)
Sb1–O2	1×	197.7(8)	Sb1–O2	1×	198.2(17)
Sb1–O5	2×	220.5(3)	Sb1–O5	2×	220.2(5)
Sb1–O8	1×	205.3(9)	Sb1–O8	1×	203.7(19)
Sb2–O1	1×	198.0(7)	Sb2–O1	1×	199.8(16)
Sb2–O3	2×	218.4(2)	Sb2–O3	2×	219.6(4)
Sb2–O7	1×	197.2(8)	Sb2–O7	1×	197.6(15)
Sb3–O1	2×	218.2(2)	Sb3–O1	2×	218.4(4)
Sb3–O2	1×	199.2(8)	Sb3–O2	1×	197.0(14)
Sb3–O3	1×	204.6(7)	Sb3–O3	1×	205.5(15)
Sb4–O4	1×	201.4(9)	Sb4–O4	1×	200.3(15)
Sb4–O5	1×	201.4(9)	Sb4–O5	1×	205.3(17)
Sb4–O8	2×	225.1(4)	Sb4–O8	2×	226.9(7)
Sb5–O4	1×	200.7(8)	Sb5–O4	1×	198.0(18)
Sb5–O9	2×	224.3(6)	Sb5–O9	2×	226.9(13)
Sb5–O10	1×	196.7(16)	Sb5–O10	1×	192(3)
Sb6–O6	1×	198.1(8)	Sb6–O6	1×	200.1(16)
Sb6–O6’	1×	206.8(6)	Sb6–O6’	1×	209.2(11)
Sb6∙∙∙O9	1×	254.1(17)	Sb6∙∙∙O9	1×	256(3)
Sb6–O10	1×	207.5(11)	Sb6–O10	1×	213.7(19)
Sb1∙∙∙Br1	1×	335.80(16)	Sb1∙∙∙I1	1×	356.8(2)
Sb1∙∙∙Br1’	2×	354.09(13)	Sb1∙∙∙I1’	2×	356.2(2)
Sb2∙∙∙Br2	2×	373.39(13)	Sb2∙∙∙I2	2×	384.6(2)
Sb3∙∙∙Br1	1×	334.11(13)	Sb3∙∙∙I1	1×	351.5(2)
Sb3∙∙∙Br1’	2×	362.23(13)	Sb3∙∙∙I1’	2×	374.7(2)
Sb4∙∙∙Br1	2×	374.84(15)	Sb4∙∙∙I1	2×	380.4(2)
Sb4∙∙∙Br2	2×	343.24(13)	Sb4∙∙∙I2	2×	358.3(2)
Sb5∙∙∙Br2	1×	332.45(16)	Sb5∙∙∙I2	1×	341.6(3)
Sb5∙∙∙Br2’	2×	343.91(13)	Sb5∙∙∙I2’	2×	364.8(2)
Sb6∙∙∙Br2	1×	343.68(18)	Sb6∙∙∙I2	1×	359.6(3)

.

3.4. Raman Spectroscopy on La₂Sb₁₂O₁₉I₄

The Raman spectrum of La₂Sb₁₂O₁₉I₄, shown in Figure 16, was recorded at an excitation wavelength of λ = 532 nm. At a wavenumber of 627 cm⁻¹, the push-pull valence vibration can be seen, which is much more pronounced in contrast to the chloride derivatives RESb₂O₄Cl (RE = Y, Sm–Lu) [3,6,12,13,14], while the valence vibration in the common mode can not be detected in the spectrum at all. This is due to the presence of isolated [Sb₄O₈]⁴⁻-ring units consisting of four vertex-connected ψ¹-tetrahedra [SbO₃]³⁻ in the RESb₂O₄Cl examples, while in La₂Sb₁₂O₁₉I₄ and La₂Sb₁₂O₁₉Br₄, endless “double-halfpipes” of antimony and oxygen with mostly tetracoordinated Sb³⁺ cations are encountered. Furthermore, the peaks at 511 and 479 cm⁻¹ can be assigned to the symmetric deformation vibrations of the vast minority of ψ¹-tetrahedra [SbO₃]³⁻. At 201, 144, 108, 74, and 61 cm⁻¹, several bands are found, which probably belong to lattice vibrations [29,30].

4. Conclusions

With the three new compounds presented here, the spectrum of lanthanum oxoantimonate(III) halides has been considerably extended. It was possible to obtain colorless single crystals of all three (monoclinic La₅Cl₃[SbO₃]₄ as well as orthorhombic La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄) and to determine their crystal structures. Furthermore, Raman measurements were performed on both structure types and compared with known rare-earth metal(III) oxoantimonate(III) halides with the composition RESb₂O₄X. Moreover, a structural comparison of tetragonal La₅F₃[SbO₃]₄ with three-dimensional and monoclinic La₅Cl₃[SbO₃]₄ with two-dimensional expression was performed, where differences and similarities could be worked out. According to X-ray powder investigations, there are more phases to consider (e.g., La₅Br₃[SbO₃]₄ with laminar and CsI₄La₂Sb_8.333O₁₄ with tubular structure characteristics, such as La₂Sb₁₂O₁₉Br₄ and La₂Sb₁₂O₁₉I₄), so further studies into the La–Sb–O–X systems seem to be necessary.