The mixed oxide MoVAlO₇

A single crystal of MoVAlO₇, vanadium aluminium molybdate, has been grown. The present structure determination is more precise than a previous powder-pattern investigation [Knorr, Jacubus, Dabrowska & Kurzawa (1998). Eur. J. Solid State Inorg. Chem. 35, 519-530]. A three-dimensional [MoAlO₆]^3n-_n network surrounds infinite strings of [VO]³⁺ groups [V-O = 1.586 (4) Å] lying in the mirror planes.

S1. Comment

A detailed study of the Mo_{1 + x}V_{2 - x}O₈ and V_{2 - x}Mo_xO₅ phases of the MoO₃–V₂O₅–V₂O₄ system has been undertaken in order to fix their homogeneity range domains, the existence of super- or modulated structures and also their magnetic and electric behaviours (Galy et al., 2001). For the former phase, synthesized at 903 K, the x limit values have been established: 0.12 < x < 0.18; in the meantime, magnetic properties have proved the presence of V⁴⁺ cations, leading to the formulation Mo⁶⁺_{1 + x}V⁵⁺_2–2xV⁴⁺_xO₈, this latter phase being a semiconductor (E_a = 0.22 eV). During the attempt to test the electrical response, a pellet was placed in an alumina crucible and heated up to 973 K; surprisingly, an extremely strong reaction occurred between the Mo_{1 + x}V_{2 - x}O₈ pellet and the crucible. In the complex mixture obtained, one small crystal, nicely prismatic and with a green colour, was detected. This crystal was quite different from the crystals of the phases of our system. It was readily studied by X-ray single-crystal techniques in order to determine its structure and therefore its chemical composition (see details of the structure determination below).

The crystal structure shows three different types of coordination for the Mo, V and Al atoms. A perspective view oriented along [010] is shown in Fig. 1. The Mo atom is surrounded by four O atoms, making a regular MoO₄ tetrahedron (see Table 1); the V atoms are inside an oxygenated VO₅ square-pyramid, while the Al atom is at the center of a regular octahedron. The VO₅ square pyramids share opposite basal edges, alternately pointing up and down, making an infinite string elongated along the helicoidal twofold axis parallel to [010]. These strings are surrounded by a kind of ring alternately built up of four MoO₄ tetrahedra and four AlO₆ octahedra through shared corners, ···Al–O2–Mo–O3–Al··· (Fig. 2). Along [010], the two other O atoms of the MoO₄ tetrahedron (O1 repeated by the mirror plane perpendicular to y axis), connect two successive AlO₆ octahedra that share edges with the VO₅ square pyramids via O4 atoms. MoVAlO₇ exhibits a three-dimensional network. We note the short V—O5 distance typical of the classical multiple bond between V⁵⁺ and oxygen in the apical position of the VO₅ square pyramid, pulling V above the basal square plane of O atoms (Table 1). The unexpected composition of such a crystal drove us to confirm immediately by solid-state synthesis the formation of this phase (see Experimental). A green powder was obtained and all the reflections of the X-ray powder pattern were indexed with the single-crystal data. The measured density of MoVAlO₇ is in good agreement with the calculated density. The MoVAlO₇ formula was then definitively established. Following this work and knowing the compound formula a search was undertaken in the literature. A structure determination by X-ray powder diffraction of the same compound was published by Knorr et al. (1998), but the previous authors were unable to grow single crystals because the phase melts incongruently at 963 K. Comparison of both structural determinations shows a reasonable agreement of the general atomic architecture. The cell parameters are similar: a = 12.7312 (6), b = 5.3763 (3), c = 8.1644 (3) Å and V = 558.8 ų (Knorr et al., 1998), but, of course, the single-crystal study allows us to obtain very accurate data on bond lengths and angles. It is then readily seen that all metal–oxygen distances are in excellent agreement with the known distances in well established structures. Especially, the short V—O distance of 1.586 (4) Å compares well with the corresponding distance in V₂O₅, i.e. 1.577 (3) Å (Enjalbert & Galy, 1986); the V—O distance of 1.48 Å found by Knorr et al. (1998) is really too short. On this basis, calculation of bond valence sums according to Brown & Altermatt (1985) gives a good agreement for the oxidation states of Mo, V and Al. These calculations strengthen also the fact that Knorr et al. (1998) had underestimated the uncertainties of their measurements by some 4σ. In both MoO₄ and AlO₆ polyhedra, the rather regular O···O distances fluctuate between O3···O2 = 2.882 (3) Å when MoO₄ connects the apices of two AlO₆ octahedra to O4···O4ⁱ = 2.464 (3) Å for the edge shared between the VO₅ square pyramid and AlO₆. This last value is associated with the smallest bond angles O4—Al—O4ⁱ = 80.0° and O4—V^iv—O4ⁱ = 77.0°, such angles being reasonably attributed to a strong repulsion between the V⁵⁺ and Al³⁺ cations. The same phenomenon appears also for the edge sharing VO₅ square pyramids, the O4···O4^iv distance diminishing to 2.425 (3) Å with O4—V—O4^iv = 79.2°. The VO₅ square pyramids are really extremely distorted; note that the O4···O4ⁱⁱ interatomic distance is 2.915 (3) Å, and that the V—O bonds towards the basal `square' plane are rather different with V—O4 = 1.822 (2) Å and V—O4ⁱⁱⁱ = 1.980 (2) Å. Such distortions explain why some authors, substituting bigger cations like Fe³⁺ and Cr³⁺ for Al³⁺, have found a triclinic distortion of the network (Le Bail et al. 1995).

S2. Experimental

MoVAlO₇ was prepared by direct interaction of a 2:1:1 stoichiometric mixture of MoO₃, V₂O₅ and Al₂O₃ (99.5% Aldrich Chemical Company). The carefully ground mixture was introduced into an open quartz tube and heated during for 24 h at 963 K. The density was determined using an Accupyc 1330 Micromeritics pycnometer.

S3. Refinement

The cell parameters and space group Pnma were clearly established. Since the cell volume was close to that of the Mo_{1 + x}V_{2 - x}O₈ phase, a rough formula of [Mo₃O₈] was tried. In the Fourier and subsequent refinements of the three main electron-density peaks, it appeared, following the values of the displacement parameters, that one peak was surely Mo, the second V, and that the remaining one was less populated in electrons. This last crystallographic site we ascribed to aluminium. The difference Fourier gave five independent O-atom positions. Refinement of the positional and anisotropic displacement parameters gave a very good R factor. The resulting formula was MoVAlO₇ with Z = 4 units per cell.

Computing details

Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL96 (Sheldrick, 1996); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL96.

Figures

	Fig. 1. Perspective view of the MoVAlO7 structure. Displacement ellipsoids are plotted at the 90% probability level.
	Fig. 2. The three-dimensional network of MoO4 and AlO6 polyhedra around the [V2O6]n^2n- infinite chains of edge-shared VO5 square pyramids.

vanadium aluminium molybdate

Crystal data

MoVAlO7	Dx = 3.394 Mg m^−3 Dm = 3.38 (2) Mg m^−3 Dm measured by helium pycnometry
Mr = 285.86	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	Cell parameters from 3861 reflections
a = 12.7360 (2) Å	θ = 2–32°
b = 5.3790 (4) Å	µ = 4.04 mm^−1
c = 8.1660 (6) Å	T = 293 K
V = 559.43 (6) Å^3	Parallelepiped, green
Z = 4	0.08 × 0.05 × 0.02 mm
F(000) = 536

Data collection

Nonius KappaCCDr diffractometer	1054 independent reflections
Radiation source: fine-focus sealed tube	878 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.067
ψ and ω scans	θmax = 32.0°, θmin = 4.1°
Absorption correction: multi-scan Blessing (1995)	h = −18→18
Tmin = 0.842, Tmax = 0.930	k = −7→7
3861 measured reflections	l = −12→12

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.035	w = 1/[σ^2(Fo^2) + (0.0488P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.083	(Δ/σ)max < 0.001
S = 1.05	Δρmax = 1.16 e Å^−3
1054 reflections	Δρmin = −2.76 e Å^−3
56 parameters	Extinction correction: SHELXL96, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0061 (12)

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

	x	y	z	Uiso*/Ueq
Mo	0.35545 (3)	0.7500	0.45506 (5)	0.00617 (14)
V	0.48684 (6)	0.2500	0.06840 (9)	0.00710 (18)
Al	0.61335 (10)	0.7500	0.26596 (16)	0.0059 (3)
O1	0.35102 (19)	0.4923 (5)	0.5845 (3)	0.0151 (5)
O2	0.4738 (2)	0.7500	0.3457 (4)	0.0137 (7)
O3	0.2482 (2)	0.7500	0.3198 (4)	0.0120 (7)
O4	0.57056 (17)	0.5208 (4)	0.0989 (3)	0.0092 (4)
O5	0.4065 (3)	0.2500	0.2167 (4)	0.0178 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mo	0.00412 (18)	0.0093 (2)	0.0051 (2)	0.000	−0.00044 (12)	0.000
V	0.0087 (3)	0.0072 (4)	0.0054 (4)	0.000	−0.0007 (3)	0.000
Al	0.0050 (5)	0.0075 (6)	0.0052 (6)	0.000	0.0005 (5)	0.000
O1	0.0179 (12)	0.0142 (13)	0.0131 (12)	0.0009 (9)	−0.0001 (9)	0.0054 (10)
O2	0.0062 (13)	0.0188 (17)	0.0161 (18)	0.000	0.0032 (13)	0.000
O3	0.0061 (13)	0.0183 (17)	0.0116 (16)	0.000	−0.0054 (12)	0.000
O4	0.0111 (10)	0.0087 (10)	0.0079 (10)	−0.0004 (8)	−0.0038 (8)	0.0000 (8)
O5	0.0204 (17)	0.0174 (18)	0.0155 (19)	0.000	0.0067 (14)	0.000

Geometric parameters (Å, º)

Mo—O1	1.744 (3)	V—V^v	2.9315 (6)
Mo—O1^i	1.744 (3)	V—V^iv	2.9315 (6)
Mo—O2	1.752 (3)	V—Al^iv	3.0138 (15)
Mo—O3	1.757 (3)	Al—O1^vi	1.843 (3)
V—O5	1.586 (4)	Al—O1^vii	1.843 (3)
V—O4^ii	1.822 (2)	Al—O3^viii	1.855 (3)
V—O4	1.822 (2)	Al—O2	1.893 (3)
V—O4^iii	1.980 (2)	Al—O4^i	1.918 (2)
V—O4^iv	1.980 (2)	Al—O4	1.918 (2)
O1—Mo—O1^i	105.29 (19)	O1^vi—Al—O1^vii	90.00 (18)
O1—Mo—O2	109.67 (10)	O1^vi—Al—O3^viii	91.27 (12)
O1^i—Mo—O2	109.67 (10)	O1^vii—Al—O3^viii	91.27 (12)
O1—Mo—O3	110.85 (10)	O1^vi—Al—O2	90.18 (12)
O1^i—Mo—O3	110.85 (10)	O1^vii—Al—O2	90.18 (12)
O2—Mo—O3	110.39 (16)	O3^viii—Al—O2	177.96 (16)
O5—V—O4^ii	105.85 (11)	O1^vi—Al—O4^i	94.98 (11)
O5—V—O4	105.85 (11)	O1^vii—Al—O4^i	174.91 (12)
O4^ii—V—O4	106.13 (14)	O3^viii—Al—O4^i	89.70 (11)
O5—V—O4^iii	106.78 (13)	O2—Al—O4^i	88.74 (12)
O4^ii—V—O4^iii	79.20 (10)	O1^vi—Al—O4	174.90 (11)
O4—V—O4^iii	143.91 (6)	O1^vii—Al—O4	94.98 (11)
O5—V—O4^iv	106.78 (13)	O3^viii—Al—O4	89.70 (11)
O4^ii—V—O4^iv	143.91 (6)	O2—Al—O4	88.74 (12)
O4—V—O4^iv	79.20 (10)	O4^i—Al—O4	80.02 (14)
O4^iii—V—O4^iv	77.02 (13)

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