The high-pressure form of cadmium vanadate, CdV₂O₆

The crystal structure of a new high-pressure modification of cadmium divanadium hexa­oxide, CdV₂O₆, was refined from X-ray single-crystal data. It contains zigzag chains of edge-sharing VO₆ octa­hedra. Octa­hedra in adjacent chains share corners and form corrugated layers. Octa­hedrally coordinated Cd atoms, which lie on twofold axes, are situated between the layers. The columbite-like structure results in a strong distortion of the CdO₆ octa­hedra which may be stabilized only at high pressure.

S1. Comment

Complex vanadium(V) oxides have received renewed interest as electrode materials and catalysts. A number of compounds with the general formula MV₂O₆ (M = Mg, Ca, Mn, Co, Ni, Cu, Zn and Cd) have been studied in the last decade (Rao & Palanna, 1996; Fuentes et al., 1999; Wei et al., 2005, 2006). Among these compounds, CuV₂O₆ (Cao et al., 2006) and CdV₂O₆ (Chen et al., 2004) have been shown to be promising cathode materials for rechargeable lithium batteries. Numerous MV₂O₆ oxides exist at ambient pressure (Rao & Palanna, 1996; Mocala & Ziolkowski, 1987; Tsuzuki et al., 1989) and their crystal structures have been reported by different authors for M = Mg (Ng & Calvo, 1972), Ca (Bouloux et al., 1972), Mn (Mueller-Buschbaum & Kobel, 1991a), Co (Jasper-Toennies & Mueller-Buschbaum, 1984; Mueller-Buschbaum & Kobel, 1991a), Ni (LeBail & Lafontaine, 1990; Mueller-Buschbaum & Kobel, 1991b), Cu (Lavaud & Galy, 1972; Calvo & Manolescu, 1973), Zn (Angenault & Rimsky, 1968; Andreetti et al.,1984) and Cd (Bouloux & Galy, 1969; Bouloux et al., 1972). Although the symmetry of the MV₂O₆ structures at ambient pressure is changed from orthorhombic to triclinic, all of them belong to the brannerite or thorutite (ThTi₂O₆) structure type. Except for M = Mn, MV₂O₆ transforms at high pressure to the orthorhombic columbite (FeNb₂O₆) structure type (Gondrand et al., 1974). The high-temperature polymorph of HgV₂O₆ also has a columbite-like structure (Mormann & Jeitschko, 2000), which is the only columbite-like MV₂O₆ vanadate stable at ambient pressure.

High-pressure polymorphs are usually denser (6–10% for MV₂O₆ phases) than their ambient pressure counterparts, and often have layered or close-packed structures. Layered structures are quite promising for applications as cathode materials. Therefore, the investigation of the high-pressure MV₂O₆ polymorphs in addition to the ambient pressure modifications may also be useful for their possible applications. However, the crystal structures of the MV₂O₆ compounds at high pressure have not been investigated yet, although their X-ray patterns have been reported (Gondrand et al., 1974). In this paper, we report the preparation of single crystals of a high-pressure modification of CdV₂O₆ and its crystal structure.

Two types of MO₆ octahedra are present in the title CdV₂O₆ structure. The VO₆ octahedra are connected via edge sharing and form zigzag chains running along the a direction (Fig. 1a). Octahedra in adjacent chains share corners and form corrugated layers separated by layers of Cd²⁺ cations. The VO₆ octahedron is strongly distorted (Fig. 2b) and the V atom is shifted towards atom O3 away from atoms O1 and O2, due to the formation of a vanadyl bond [1.6548 (10) Å]. As a result, the V—O2 bond opposite the vanadyl bond is elongated to 2.3505 (10) Å. The O2—V—O3 angle is 167.72 (4)°. Bond-valence sum (BVS) calculations gave values of 5.23 and 4.87 for six- and five-coordinated V, respectively (Schindler et al., 2000). This type of distortion of the coordination polyhedron is typical for pentavalent V, and similar distances and angles for the VO₆ octahedron were found, for instance, in the columbite-like structure of HgV₂O₆ (Mormann & Jeitschko, 2000). The CdO₆ octahedra are linked by edge sharing into a zigzag chain running along the a direction. The chains are not connected to each other (Fig. 1b). The CdO₆ octahedra are also strongly distorted (Fig. 2a) and have three pairs of Cd—O distances, 2.1961 (10), 2.3071 (10) and 2.4311 (10) Å. The largest O—Cd—O angle is 155.82 (4)°. BVS calculation (Brown & Altermatt, 1985; Brese & O'Keeffe, 1991) gives a value of 2.06, in spite of the strong distortion of the polyhedron. One may speculate that this distortion is a consequence of the almost pyramidal coordination of the V atom.

One may suggest that the strong distortion of the coordination polyhedra in the high-pressure structure is a reason for the instability of this modification at ambient pressure. In the brannerite-like structure, the Cd atom has an almost regular octahedral coordination with a small difference between the equatorial and apical Cd—O distances (2.287 and 2.199 Å, respectively). The coordination polyhedron of the V atom should be considered as a trigonal bipyramid with two vanadyl bonds of 1.687 and 1.708 Å (Zavalij & Whittingham, 1999). In the high-pressure structure, the V atoms have six O neighbours. However, the sixth V—O2 distance is 2.3505 (10) Å, and it is possible to use the τ parameter (Addison et al., 1984) to distinguish between the VO₅ square-pyramidal and trigonal–bipyramidal geometries. The τ parameter is 0.12, indicating that, if the sixth O2 atom is ignored, the VO₅ polyhedron is a square pyramid. In HgV₂O₆, the VO₆ polyhedron (τ = 0.17) is very similar to that in CdV₂O₆ but the sixth V–O bond is longer (2.442 Å). The main difference between the two structures arises from the coordination of the Hg atom, with two very short (2.040 Å) and four long (2.643 and 2.677 Å) Hg—O distances. Such a coordination stabilizes a columbite-like structure, in spite of the square-pyramidal coordination of the V atom. One should note that, in other known columbite-like compounds, both cations have a nearly regular octahedral coordination. This is not the case for CdV₂O₆ and, although the cation–cation distances are very close in both modifications, the O environments for these cations are significantly different. Thus, one may conclude that the MV₂O₆ columbite-like structure results in a strong distortion of the MO₆ octahedra which may be stabilized only at high pressure.

Related literature

For related literature, see: Addison et al. (1984); Andreetti et al. (1984); Angenault & Rimsky (1968); Bouloux & Galy (1969); Bouloux et al. (1972); Brese & O'Keeffe (1991); Brown & Altermatt (1985); Calvo & Manolescu (1973); Cao et al. (2006); Chen et al. (2004); Fuentes et al. (1999); Gondrand et al. (1974); Jasper-Toennies & Mueller-Buschbaum (1984); Lavaud & Galy (1972); LeBail & Lafontaine (1990); Mocala & Ziolkowski (1987); Mormann & Jeitschko (2000); Mueller-Buschbaum & Kobel (1991a, 1991b); Ng & Calvo (1972); Rao & Palanna (1996); Schindler et al. (2000); Tsuzuki et al. (1989); Wei et al. (2005, 2006); Zavalij & Whittingham (1999).

S2. Experimental

A mixture of CdO (99.9%) and V₂O₅ (99.9%) in a 1:1 ratio was placed in a gold capsule and treated at 6 GPa and 1473 K in a belt-type high-pressure apparatus for 60 min (heating rate of 120 K min^-1). After heat treatment, the sample was quenched to room temperature and the pressure was released slowly. The product consisted of small black single crystals suitable for X-ray crystal structure analysis. Powder X-ray diffraction revealed that the crystals are CdV₂O₆.

S3. Refinement

The structure was refined isotropically to R = 0.032 and anisotropically to R = 0.016. Difference Fourier syntheses revealed the highest residual peaks in the vicinity of the Cd and V sites. Anharmonic displacement parameters based on the Gram–Charlier expansion of the structure factor were refined up to the fourth order. Although the depths of negative regions in the probability density function maps were about 3–6%, only a few parameters had a significance level above 3σ and the reduction of the R factor was about 0.001. Therefore, in the final refinement cycles, all atoms were refined anisotropically. At the final stage, the occupancies of all sites were refined in turn, with deviations from unity less than 3σ in all cases. Fixed full occupancies were then used for all atoms. The use of the I > 2σ(I) cut-off instead of I > 3σ(I) added only eight extra observed reflections and raised the R index only slightly (0.018).

Computing details

Data collection: CAD-4 Manual (Enraf–Nonius, 1988); cell refinement: CAD-4 Manual; data reduction: JANA2000 (Petříček & Dušek, 2000); program(s) used to solve structure: SIR2002 (Burla et al., 2000); program(s) used to refine structure: JANA2000; molecular graphics: ATOMS for Windows (Dowty, 1998); software used to prepare material for publication: JANA2000.

Figures

	Fig. 1. The crystal structure of CdV2O6. (a) Corrugated layers of VO6 octahedra. (b) Chains of CdO6 octahedra.
	Fig. 2. Coordination polyhedra for (a) the Cd atoms and (b) the V atoms. [Symmetry codes: (i) ?; (ii) ?; (iii) ?; (v) ?; (vi) ? Please complete]

cadmium divanadium hexaoxide

Crystal data

CdV2O6	F(000) = 568
Mr = 310.29	Dx = 5.159 Mg m^−3
Orthorhombic, Pnab	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2bc 2n	Cell parameters from 23 reflections
a = 4.9495 (6) Å	θ = 18.5–24.2°
b = 5.6949 (7) Å	µ = 9.79 mm^−1
c = 14.1692 (17) Å	T = 293 K
V = 399.39 (8) Å^3	Plate, black
Z = 4	0.22 × 0.11 × 0.06 mm

Data collection

Enraf–Nonius CAD-4 diffractometer	1349 reflections with I > 3σ(I)
Radiation source: X-ray sealed tube	Rint = 0.020
Equatorial mounted graphite monochromator	θmax = 46.0°, θmin = 2.9°
ω scans	h = 0→10
Absorption correction: gaussian (JANA2000; Petříček & Dušek, 2000)	k = −3→11
Tmin = 0.314, Tmax = 0.565	l = −20→28
4388 measured reflections	2 standard reflections every 120 min
1727 independent reflections	intensity decay: 0.0%

Refinement

Refinement on F	Weighting scheme based on measured s.u.'s w = 1/[σ^2(F) + 0.000324F^2]
R[F^2 > 2σ(F^2)] = 0.016	(Δ/σ)max = 0.002
wR(F^2) = 0.028	Δρmax = 0.58 e Å^−3
S = 1.19	Δρmin = −0.49 e Å^−3
1349 reflections	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
43 parameters	Extinction coefficient: 0.099926

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cd	0.75	0.77230 (3)	1.0	0.00857 (4)
V	0.72263 (5)	0.31327 (4)	0.833072 (15)	0.00574 (4)
O1	0.5083 (2)	0.57752 (18)	0.89624 (6)	0.00838 (19)
O2	0.3843 (2)	0.35145 (19)	0.75452 (6)	0.00830 (18)
O3	0.6127 (2)	0.11338 (19)	0.90865 (7)	0.00938 (19)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd	0.00772 (6)	0.01005 (7)	0.00793 (6)	0	−0.00074 (3)	0
V	0.00534 (7)	0.00615 (7)	0.00572 (8)	0.00057 (5)	0.00023 (5)	−0.00066 (5)
O1	0.0076 (3)	0.0088 (4)	0.0087 (3)	0.0021 (3)	−0.0022 (3)	−0.0020 (3)
O2	0.0076 (3)	0.0093 (3)	0.0080 (3)	−0.0002 (3)	−0.0023 (3)	−0.0010 (3)
O3	0.0086 (3)	0.0098 (4)	0.0097 (3)	−0.0001 (3)	0.0011 (3)	0.0018 (3)

Geometric parameters (Å, º)

Cd—O1	2.1961 (10)	V—O1	2.0472 (10)
Cd—O1^i	2.1961 (10)	V—O1^iv	1.7853 (10)
Cd—O3^ii	2.4311 (10)	V—O2	2.0225 (10)
Cd—O3^iii	2.3071 (10)	V—O2^vi	1.7494 (10)
Cd—O3^iv	2.3071 (10)	V—O2^iv	2.3505 (10)
Cd—O3^v	2.4311 (10)	V—O3	1.6548 (10)
O1—Cd—O1^i	119.32 (4)	O3^v—Cd—O3^iv	81.71 (3)
O1—Cd—O3^ii	83.97 (4)	O1—V—O1^iv	86.28 (4)
O1—Cd—O3^iii	95.41 (4)	O1—V—O2	74.49 (4)
O1—Cd—O3^iv	101.00 (3)	O1—V—O2^vi	160.28 (4)
O1—Cd—O3^v	155.82 (4)	O1—V—O2^iv	77.66 (4)
O1^i—Cd—O1	119.32 (4)	O1—V—O3	93.01 (5)
O1^i—Cd—O3^ii	155.82 (4)	O1^iv—V—O1	86.28 (4)
O1^i—Cd—O3^iii	101.00 (3)	O1^iv—V—O2	153.41 (5)
O1^i—Cd—O3^iv	95.41 (4)	O1^iv—V—O2^vi	100.38 (5)
O1^i—Cd—O3^v	83.97 (4)	O1^iv—V—O2^iv	71.63 (4)
O3^ii—Cd—O3^iii	81.71 (3)	O1^iv—V—O3	100.11 (5)
O3^ii—Cd—O3^iv	72.14 (3)	O2—V—O2^vi	92.63 (4)
O3^ii—Cd—O3^v	73.93 (3)	O2—V—O2^iv	86.21 (4)
O3^iii—Cd—O3^ii	81.71 (3)	O2—V—O3	99.09 (5)
O3^iii—Cd—O3^iv	147.22 (4)	O2^vi—V—O2	92.63 (4)
O3^iii—Cd—O3^v	72.14 (3)	O2^vi—V—O2^iv	86.79 (4)
O3^iv—Cd—O3^ii	72.14 (3)	O2^vi—V—O3	103.92 (5)
O3^iv—Cd—O3^iii	147.22 (4)	O2^iv—V—O2	86.21 (4)
O3^iv—Cd—O3^v	81.71 (3)	O2^iv—V—O2^vi	86.79 (4)
O3^v—Cd—O3^ii	73.93 (3)	O2^iv—V—O3	167.72 (4)
O3^v—Cd—O3^iii	72.14 (3)

Symmetry codes: (i) −x+3/2, y, −z+2; (ii) x, y+1, z; (iii) −x+1, −y+1, −z+2; (iv) x+1/2, −y+1, z; (v) −x+3/2, y+1, −z+2; (vi) x+1/2, −y+1/2, −z+3/2.