Synthesis, structure and characterization of two new antimony oxides–LaSb₃O₉ and LaSb₅O₁₂: Formation of LaSb₅O₁₂ from the reaction of LaSb₃O₉ with Sb₂O₃

Abstract

Two new lanthanum antimony oxides, LaSb₃O₉ and LaSb₅O₁₂, have been synthesized as bulk phase powders and single crystals by solid state reactions using La₂O₃ (or La(NO₃)₃·xH₂O), Sb₂O₃ and Sb₂O₅ as reagents. The structures of LaSb₃O₉ and LaSb₅O₁₂ were determined by single crystal X-ray diffraction. LaSb₃O₉ contains exclusively Sb⁵⁺ cations, whereas LaSb₅O₁₂ contains both Sb³⁺ and Sb⁵⁺ cations. LaSb₃O₉ has a novel three-dimensional framework consisting of edge- and corner-shared Sb⁵⁺O₆ octahedra. LaSb₅O₁₂ exhibits a layered crystal structure consisting of corner-shared Sb⁵⁺O₆ octahedra. The Sb³⁺O₃ groups cap the SbO₆ layers from above and below. The Sb³⁺ cations are in asymmetric coordination environments attributable to their stereo-active lone-pair. We also report on the reaction of three-dimensional LaSb₃O₉ with Sb₂O₃ to form the two-dimensional LaSb₅O₁₂. Infrared, thermogravimetric and dielectric analyses are also presented.

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Introduction

A rich structural chemistry has been observed in mixed metal oxides containing cations with non-bonded electron pairs (Pb²⁺, Bi³⁺, Sb³⁺, Te⁴⁺ and Se⁴⁺).^1–5 These cations are found in a variety of asymmetric coordination environments attributable to their stereo-active lone-pair. The lone-pair is thought to be the result of a second-order Jahn–Teller (SOJT) distortion.^6–10 With these cations, a SOJT distortion reduces the energy between the highest occupied (HOMO) s-orbital and the lowest unoccupied (LUMO) p-orbital through s–p mixing.^11–14 Combining this variable coordination geometry with octahedral or tetrahedral moieties produces a great deal of interesting framework architecture. We recently reported the syntheses, structures, and optical behavior of a few materials that contain SOJT distorted cations.^15–17 With these ideas in mind, we investigated materials in the La–Sb–oxide system. Attributable to its structural versatility, compounds containing Sb³⁺ exhibit a great deal of interesting materials properies including magnetic,¹⁸ thermoelectric,¹⁹ second-harmonic generating,²⁰ and dielectric behavior.²¹ With respect to rare earth antimony oxides, a few materials have been reported, namely Pr₃SbO₇,²² La₃Li₅Sb₂O₁₂,²³ Ln₃Sb₅O₁₂ (Ln = Pr, Nd, Sm, Gd, Yb or Eu),^24–27 and LaSbO₄.²⁸ Except for Ln₃Sb₅O₁₂^24–27 all of these materials contain Sb⁵⁺. Structurally the materials are very different, with Pr₃SbO₇²² containing one-dimensional SbO₆ chains, whereas La₃Li₅Sb₂O₁₂²³ exhibits discrete SbO₆ octahedra. Only Ln₃Sb₅O₁₂^24–27 has a three-dimensional structure, consisting of asymmetric SbO₃ and SbO₄ polyhedra that are linked through oxygen atoms. For LaSbO₄, only a powder X-ray diffracation pattern has been reported. In this paper, we report the synthesis, structure and characterization of two new materials, LaSb₃O₉ and LaSb₅O₁₂ (LaSb₂³⁺Sb₃⁵⁺O₁₂). In addition, we report on the formation of LaSb₅O₁₂ through the reaction of LaSb₃O₉ and Sb₂O₃.

Experimental

Synthesis

Sb₂O₃ (Alfa Aesar, 99.6%), Sb₂O₅ (Aldrich, 99.995%), La(NO₃)₃·xH₂O (Aldrich, 99.9%) and TeO₂ (Aldrich, 99%) were used as received. La₂O₃ (Aldrich, 99.9%) was dried overnight at 1000 °C before being used. α-Sb₂O₄ was synthesized by heating Sb₂O₃ in air at 600 °C for 24 h and the purity was confirmed by powder X-ray diffraction.

Synthesis of LaSb₃O₉. Single crystals of LaSb₃O₉ were prepared using TeO₂ as a flux. 0.192 g (0.59 × 10⁻³ mol) of La₂O₃, 0.190 g (0.59 × 10⁻³ mol) of Sb₂O₅ and 0.563 g (3.53 × 10⁻³ mol) of TeO₂ were ground with an agate mortar and pestle and introduced into a gold tube that was subsequently sealed. The tube was heated to 900 °C for 24 h and then cooled to 500 °C at 6 °C h⁻¹ before being quenched to room temperature. Pale yellow crystals of LaSb₃O₉ were recovered along with TeO₂ and unknown amorphous phases from the gold tube. The isolated yield was approximately 20% based on the weight of Sb₂O₅.

Polycrystalline LaSb₃O₉ was prepared by standard solid-state techniques. 1.083 g (3.33 × 10⁻³ mol) of La(NO₃)₃·xH₂O and 1.618 g (5.00 × 10⁻³ mol) of Sb₂O₅ were ground thoroughly with an agate mortar and pestle and pressed into a pellet. The pellet was heated to 350 °C for 3 h, and then to 900 °C for 48 h in air and cooled to room temperature with two intermittent re-grindings. The use of La(NO₃)₃·xH₂O was critical in preventing the reduction of Sb⁵⁺ to Sb³⁺. The powder X-ray diffraction pattern on the resultant white powder is in good agreement with the generated pattern from the single crystal data (see ESI†).

Synthesis of LaSb₅O₁₂. The LaSb₅O₁₂ crystals were grown by combining 0.225 g (0.69 × 10⁻³ mol) of La₂O₃ and 1.275 g (4.15 × 10⁻³ mol) of α-Sb₂O_4. The mixture was ground with an agate mortar and pestle and introduced into a platinum tube that was subsequently sealed. The tube was heated to 1000 °C for 24 h, cooled to 800 °C at 6 °C h⁻¹, and then cooled to room temperature at 60 °C h⁻¹. Colorless blocks of LaSb₅O₁₂ crystals were recovered from the tube in 60% yield based on the weight of α-Sb₂O₄.

Pure LaSb₅O₁₂ powder was synthesized by combining 0.362 g (1.11 × 10⁻³ mol) of La₂O₃, 0.648 g (2.22 × 10⁻³ mol) of Sb₂O₃ and 1.078 g (3.33 × 10⁻³ mol) of Sb₂O₅. The mixture was thoroughly ground and pressed into a pellet. The pellet was wrapped in platinum foil, introduced into a fused silica tube that was sealed under vacuum. The tube was heated to 750 °C for 24 h, 850 °C for 48 h, 900 °C for 24 h, and then cooled to room temperature at a rate of 10 °C min⁻¹ with four intermediate re-grindings. The powder X-ray diffraction pattern on the resultant white powder indicated the material was single-phase and in good agreement with the generated pattern from the single crystal data (see ESI†).

Crystallographic determination

A pale yellow block (0.04 × 0.16 × 0.40 mm) for LaSb₃O₉ and a colorless block (0.12 × 0.15 × 0.20 mm) for LaSb₅O₁₂ were used for single crystal data analyses. Room-temperature intensity data were collected on a Siemens SMART diffractometer equipped with a 1K CCD area detector using graphite-monochromated Mo-Kα radiation. A hemisphere of data was collected using a narrow-frame method with scan widths of 0.30° in omega, and an exposure time of 30 s frame⁻¹. The first 50 frames were re-measured at the end of the data collection to monitor instrument and crystal stability. The maximum correction applied to the intensities was <1%. The data were integrated using the Siemens SAINT program,²⁹ with the intensities corrected for Lorentz, polarization, air absorption, and absorption attributable to the variation in the path length through the detector faceplate. Psi-scans were used for the absorption correction on the hemisphere of data. The data were solved and refined using SHELXS-97 and SHELXL-97, respectively.^30,31 All of the metal atoms were refined with anisotropic thermal parameters and converged for I > 2σ(I). All calculations were performed using the WinGX-98 crystallographic software package.³² Crystallographic data, atomic coordinates and displacement parameters, and selected bond distances for LaSb₃O₉ and LaSb₅O₁₂ are given in Tables 1–4. The X-ray powder diffraction data were collected on a Scintag XDS2000 diffractometer at room temperature (Cu-Kα radiation, θ–θ mode, flat plate geometry) in the 2θ range 5–60° with a step size of 0.02°, and a step time of 2 s.

Table 1 Crystallographic data for LaSb₃O₉ and LaSb₅O₁₂

Formula	LaSb3O9	LaSb5O12
a R 1 = Σ||Fo| − |Fc||/Σ|Fo|. b w R 2 = [Σw(Fo^2 − Fc^2)^2/Σw(Fo^2)^2]^1/2.
Formula weight	648.16	939.66
Crystal system	Orthorhombic	Trigonal
Space group	Cmcm (no. 63)	R3̄m (no. 166)
a/Å	6.5505(11)	7.2707(7)
b/Å	8.8941(11)	7.2707(7)
c/Å	11.8274(17)	16.4673(12)
α/°	90	90
β/°	90	90
γ/°	90	120
V/Å^3	689.07(17)	753.89(12)
Z	4	3
μ(Mo-Kα)/mm^−1	17.735	17.466
Refl. collected/unique	2040/450	1549/251
R int	0.0798	0.0472
R 1 ^a [I > 2σ(I)]	0.0438	0.0230
wR 2 ^b [I > 2σ(I)]	0.1261	0.0596

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Table 2 Atomic coordinates for LaSb₃O₉

Atom	x	y	z	U eq ^a/Å^2	Occupancy
a U eq is defined as one third of the trace of the orthogonalized Uij tensor. b All oxygen atoms were refined isotropically.
La(1)	0	0.2449(1)	0.75	0.012(1)	1.0
Sb(1)	0	0.5	0.5	0.007(1)	1.0
Sb(2)	0	0.1217(1)	0.4084(1)	0.007(1)	1.0
O(1)	−0.1968(10)	0.6111(6)	0.4014(5)	0.010(1)^b	1.0
O(2)	0	0.3336(10)	0.3849(8)	0.012(2)	1.0
O(3)	0	0.1026(10)	0.5746(8)	0.008(2)	1.0
O(4)	0	0.0605(13)	0.25	0.007(2)	1.0

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Table 3 Atomic coordinates for LaSb₅O₁₂

Atom	x	y	z	U eq ^a/Å^2	Occupancy
a U eq is defined as one third of the trace of the orthogonalized Uij tensor.
La(1)	0.3333	0.6667	0.1667	0.007(1)	1.0
Sb(1)	0	0	0.1517(1)	0.007(1)	1.0
Sb(2)	0.1667	0.3333	0.3333	0.006(1)	1.0
O(1)	0.2513(10)	0.1256(5)	0.3657(3)	0.010(1)	1.0
O(2)	0.1315(5)	0.2629(10)	0.2168(3)	0.011(1)	1.0

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Table 4 Selected bond distances (Å) for LaSb₃O₉ and LaSb₅O₁₂

LaSb3O9		LaSb5O12
Sb(1)–O(1) × 4	2.000(6)	Sb(1)–O(2) × 3	1.972(6)
Sb(1)–O(2) × 2	2.011(10)	Sb(2)–O(1) × 4	1.968(2)
Sb(2)–O(1) × 2	1.989(7)	Sb(2)–O(2) × 2	1.970(6)
Sb(2)–O(2)	1.906(9)
Sb(2)–O(3)	1.972(10)
Sb(2)–O(3)	2.005(9)
Sb(2)–O(4)	1.951(3)

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CCDC reference numbers 218480 and 218481.

See http://www.rsc.org/suppdata/jm/b3/b307496j/ for crystallographic data in CIF or other electronic format.

Characterization

Infrared spectra were recorded on a Matteson FTIR 5000 spectrometer in the 400–4000 cm⁻¹ range, with the sample pressed between two KBr pellets. Thermogravimetric analyses were carried out on a TGA 2950 (TA Instruments). The samples were contained within platinum crucibles and heated at a rate of 10 °C min⁻¹ from room temperature to 900 °C in nitrogen. The dielectric constant (κ) and quality factor (Q = 1/tan δ) measurements were performed using a HP4192A impedance analyzer operating at 1 MHz. Polycrystalline LaSb₃O₉ and LaSb₅O₁₂ were pressed into 1.2 cm diameter and 0.15 cm thick pellets and sintered at 900 °C for 24 h. The pellets had a density 90% of theoretical. A conducting silver paste was applied to the pellet surfaces for electrodes and cured at 400 °C. The temperature dependence of the dielectric constants (TCK) was measured, between −20 and 100 °C, by placing the pellets in a Linkam THMSE600 hot stage.

Results and discussion

Structures

LaSb₃O₉ exhibits a three-dimensional crystal structure consisting of layers of edge- and corner-shared SbO₆ octahedra. Each layer is connected through an oxygen atom to form the three-dimensional topology. As seen in Fig. 1(a), six-membered ring channels are formed that run parallel to the a-axis. The La³⁺ cations reside in these channels. Within each layer, two [Sb(2)O_6/2]⁻ octahedra share edges through O(3), whereas an [Sb(1)O_6/2]⁻ octahedron shares corners with four [Sb(2)O_6/2]⁻ octahedra through O(1) and O(2) (see Fig. 1(b)). The layers are connected by a single oxygen atom, O(4). The Sb–O bond distances lie in the range 1.906(9)–2.008(10) Å. The La³⁺ cation is in a seven-fold coordination environment with La–O contacts ranging from 2.431(10) to 2.717(12) Å. In connectivity terms, LaSb₃O₉ can be formulated as consisting of a {3[SbO_6/2]⁻}³⁻ anionic framework with charge balance maintained by the La³⁺ cation. Bond valence calculations^33,34 on LaSb₃O₉ resulted in values of 2.66 and 5.10 for La³⁺ and Sb⁵⁺, respectively.

Fig. 1 (a) Ball-and-stick diagram of LaSb₃O₉ in the bc plane. Note the six-membered ring parallel to the a-axis in which the La³⁺ cations are found. (b) Ball-and-stick diagram of local environments in LaSb₃O₉ indicating the connectivity of Sb(1)O₆ and Sb(2)O₆ units.

LaSb₅O₁₂ (LaSb₂³⁺Sb₃⁵⁺O₁₂) exhibits a two-dimensional layered crystal structure composed of corner-shared Sb⁵⁺O₆ octahedra. The asymmetric Sb³⁺O₃ groups cap the Sb⁵⁺O₆ octahedra from above and below and serve as intra-layer linkers (see Fig. 2). Each Sb⁵⁺O₆ octahedron is connected to four additional Sb⁵⁺O₆ octahedra and two Sb³⁺O₃ trigonal pyramids, whereas each Sb³⁺O₃ pyramid is connected to three Sb⁵⁺O₆ octahedra. The Sb⁵⁺–O bond distances lie in the range 1.968(2)–1.970(6) Å, whereas Sb³⁺–O has a unique bond distance of 1.972(6) Å. The La³⁺ cation is in a twelve-fold coordination environment with La–O contacts ranging from 2.673(6) to 2.720(6) Å. The non-bonded electron pair on the Sb³⁺ cation points toward the La³⁺. In connectivity terms, the structure can be described as a two-dimensional layer of {[Sb³⁺O_3/2]⁰3[Sb⁵⁺O_6/2]⁻}³⁻. Charge neutrality is maintained by the La³⁺ cation. Bond valence calculations^33,34 resulted in values of 2.91, 3.01 and 5.58 for La³⁺, Sb³⁺ and Sb⁵⁺, respectively.

Fig. 2 Ball-and-stick diagram of LaSb₅O₁₂ in the ac plane. Note that the Sb³⁺O₃ groups serve as intra-layer linkers, by capping the Sb⁵⁺O₆ layer from above and below.

Interestingly, the structural motif of LaSb₅O₁₂ is similar to that of A₂TeW₃O₁₂ (A = Rb or Cs).³⁵ This similarity can be better understood by re-writing LaSb₅O₁₂ as LaSb₂³⁺Sb₃⁵⁺O₁₂. Thus both LaSb₂Sb₃O₁₂ and A₂TeW₃O₁₂ have a “M₃O₁₂” sub-structure (M = Sb⁵⁺ or W⁶⁺), with the three remaining cations in the formula as either “LaSb₂” or “A₂Te” for LaSb₂Sb₃O₁₂ and A₂TeW₃O₁₂, respectively. Both LaSb₂Sb₃O₁₂ and A₂TeW₃O₁₂ contain a layer of MO₆ octahedra, Sb⁵⁺O₆ and W⁶⁺O₆, respectively, that is similar to hexagonal-WO₃.³⁶ However, in LaSb₂Sb₃O₁₂ these layers are anionic, [Sb⁵⁺O_6/2]⁻, whereas in A₂TeW₃O₁₂ the analogous layers are neutral, [W⁶⁺O_6/2]⁰. This difference in charge produces different atomic connectivities in the crystal structures. For LaSb₂Sb₃O₁₂, the Sb³⁺O₃ groups link to the Sb⁵⁺O₆ layer on both sides, above and below (see Fig. 3), whereas in A₂TeW₃O₁₂, the Te⁴⁺O₃ groups connect to the WO₆ layer only on one side. This ‘single-sided’ connectivity of the TeO₃ groups results in a non-centrosymmetric polar structure, compared with the ‘above and below’ linking of the SbO₃ groups that produces a centrosymmetric structure. Between the MO₆ layers (M = Sb⁵⁺ or W⁶⁺) are the La³⁺ or A⁺ (A = Rb or Cs) cations, respectively.

Fig. 3 Ball-and-stick (top) and polyhedral (bottom) representation of one-layer of LaSb₅O₁₂ in the ab plane.

If we write LaSb₅O₁₂ as LaSb₂Sb₃O₁₂ and ‘subtract’ Sb₂O₃ from the stoichiometry, we are left with LaSb₃O₉. Therefore reacting Sb₂O₃ with LaSb₃O₉ should produce LaSb₅O₁₂. A stoichiometric mixture of LaSb₃O₉ and Sb₂O₃ was heated in an evacuated fused silica tube to 900 °C for 24 h and 1000 °C for an additional 24 h. Powder XRD measurements on the reaction product revealed a diffraction pattern consistent with LaSb₅O₁₂. Interestingly, this reaction is irreversible. LaSb₅O₁₂ is stable up to 1100 °C, with additional heating resulting in decomposition to LaSbO₄.²⁸

Characterization

Infrared spectroscopy. The infrared spectra of both materials revealed Sb–O (730–860 cm⁻¹), Sb–O–Sb (507–690 cm⁻¹) and O–Sb–O (420–480 cm⁻¹) vibrations. The infrared vibrations and assignments are given in Table 5. The assignments are consistent with those previously reported.³⁷

Table 5 Infrared vibrations (cm⁻¹) for LaSb₃O₉ and LaSb₅O₁₂

	ν(Sb–O)	ν(Sb–O–Sb)	δ(O–Sb–O)
LaSb3O9	860	690	484
	804	653	462
	736	553
		526
		507
LaSb5O12	794	699	489
	755		466
			422

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Thermogravimetric analysis. The thermal behavior of LaSb₃O₉ and LaSb₅O₁₂ were investigated using thermogravimetric analysis. No weight losses or phase changes were observed up to 900 °C for both materials. Heating LaSb₃O₉ and LaSb₅O₁₂ to 1200 °C resulted in both materials decomposing to crystalline LaSbO₄.²⁸

Dielectric measurements. Since the reported materials contain polarizable cations, we felt it would be interesting to investigate their bulk dielectric properties. The dielectric constants (κ) for LaSb₃O₉ and LaSb₅O₁₂ are 20.5 and 28.7, respectively. Interestingly, the quality factor (Q = 1/tan δ) and temperature dependence of the dielectric constant (TCK) for LaSb₃O₉ are superior to LaSb₅O₁₂ (see Table 6). Additional measurements are underway in order to ascertain the origin of this dielectric behavior.

Table 6 Dielectric constant (κ), quality factor (Q) and temperature coefficient of the dielectric constant (TCK) for LaSb₃O₉ and LaSb₅O₁₂ at 1 MHz and 20 °C

	κ ^a	Q (1/tan δ)^b	TCK^c/ppm °C^−1
a κ = Dielectric constant. b Q = 1/tan δ. c TCK = [(κ100 − κ−20)/κ40]/120
LaSb3O9	20.49	32.15	32.5
LaSb5O12	28.87	2.90	116.7

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Conclusions

We have synthesized, as both bulk phases and single crystals, two new lanthanum antimony oxides, LaSb₃O₉ and LaSb₅O₁₂. The materials were characterized by single crystal and powder diffraction as well as infrared, thermogravimetric and dielectric analyses. LaSb₃O₉ has a three-dimensional crystal structure, whereas LaSb₅O₁₂ exhibits a layered motif. We have also demonstrated that LaSb₅O₁₂ can be formed by the stoichiometric reaction between LaSb₃O₉ and Sb₂O₃. We continue to explore the synthesis of new antimony oxides, as well as investigate the incorporation of other cations with stereo-active lone-pairs into new materials.

Acknowledgements

We thank the Robert A. Welch Foundation for support. This work was also supported by the NSF-Career Program through DMR-0092054 and an acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. P. S. H. is a Beckman Young Investigator.

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Footnote

† Electronic supplementary information (ESI) available: Powder X-ray diffraction patterns (calculated and experimental). See http://www.rsc.org/suppdata/jm/b3/b307496j/

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