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The crystal structure of the new complex vanadium oxide Na6Mg2(VO4)2(V2O7) was solved from X-ray single-crystal data. The structure contains VO4 tetrahedra and MgO6 octahedra, linked by corners and forming a complex three-dimensional framework. A half of the VO4 tetrahedra are connected only to MgO6 octahedra, whereas the others are corner-sharing, forming V2O7 pyrovanadate groups with statistically random orientations. One unique Mg atom is located at an inversion centre, while the other Mg atom, one unique V atom and five unique O atoms lie on mirror planes.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104005384/iz1039sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270104005384/iz1039Isup2.hkl
Contains datablock I

Comment top

Complex vanadium oxides have drawn the attention of researchers because of their possible application as secondary current sources and catalysts in oxidation processes (Schindler et al., 2000; Zavalij & Wittingham, 1999). The variety of vanadium oxidation states and coordination polyhedra gives numerous opportunities for the synthesis of new compounds.

Until now, only two compounds have been reported in the Na/Mg/V/O system. The crystal structure of NaMg4(VO4)3 (Murashova et al., 1988) contains isolated V+5O4 tetrahedra and MgO6 octahedra, linked in a three-dimensional framework. Na and Mg atoms adopt ordered positions in the structural interstices. The crystal structure of Na6Mg3V4O16 (Slobodin et al., 1987) is unknown; moreover, the X-ray diffraction powder pattern of this compound was not indexed.

The structure of the title compound, Na6Mg2V4O15, is shown in Fig. 1. The three-dimensional framework is built up of corner-sharing V5+O4 tetrahedra and MgO6 octahedra. The MgO6 octahedra are close to regular, with Mg—O bonds in the range 2.07–2.25 Å. Every octahedron is corner-linked to six VO4 tetrahedra. The Na1, Na2 and Na3 sites have five O-atom neighbours, and Na4 has six, with an Na—O bond-length range of 2.29–2.67 Å.

There are three symmetry-independent vanadium sites in the structure, all of which are tetrahedrally coordinated and contain V5+ cations. The bond-valence sums (BVSs) for the V1-, V2- and V3-atom positions are 5.05, 5.18 and 4.77, respectively. The V1O4 and V2O4 tetrahedra are practically undistorted and have four V—O distances close to 1.70 Å, which is typical for vanadate groups. The V3O4 tetrahedron is strongly distorted, with a noticeable elongation of the V3—O9 bond length to 1.933 Å. Atom V3 partially (1/2) occupies an 8 d site, which results from the splitting of a 4c site along the b axis, the separation between atoms V3a and V3b being 1.064 Å. Only one of the two V3O4 tetrahedra can be occupied at any one time.

All tetrahedra in the structure are corner-linked. The V1O4 tetrahedron has four neighboring MgO6 octahedra, while the V2O4 group shares three corners with MgO6 octahedra and one with the V3O4 tetrahedron. The latter is also connected to two Mg1O6 octahedra. The coordinating atom, O2, belongs only to the V3O4 tetrahedra. Fig. 2 shows that the the corner-linked V2O4 and V3aO4 (or V3bO4) tetrahedra together form randomly oriented V2O7 pyrovanadate groups. Hence the structural formula for the title compound can be written as Na6Mg2(V2O7)(VO4)2.

The random orientation of the pyrovanadate groups explains the statistical populations on the coordinated Na3 and Na4 sites, which are only half occupied. We suggest that these atoms should be situated on the opposite side (of what?) to any V3aO4 or V3bO4 tetrahedron present.

Although a trigonal bipyramidal coordination can be realised for a V5+ cation, no such configuration is realised in the Na6Mg2V4O15 structure. A shift of atom V3 to the center of the O4/O2/O5 plane of a hypothetical bipyramid (4c position) results in enormous V—O9 distances (about 2.46 Å), which can be considered to be non-bonding. A BVS calculation indicates a vanadium oxidation state of +5.46 for this configuration, which is much higher than the maximum possible vanadium valence.

The crystal structure of Na6Mg2V4O15 is similar to that of Na2Ca6(Si2O7)(SiO4)2 (apace group P21/c; Armbruster & Roethlisberger, 1990). The latter has a monoclinic lattice with cell parameters close to those of Na6Mg2V4O15. In contrast to the title structure, Na2Ca6(Si2O7)(SiO4)2 has one SiO4 tetrahedra, the Si2O7 groups are placed in an orderly manner and the A cations (Na and Ca) jointly occupy their positions.

The theoretical X-ray powder pattern for Na6Mg2V4O15, calculated from single-crystal data, coincides well with the powder data reported by Slobodin et al. (1987), who assumed the composition Na6Mg3V4O16. Taking into account the similarity of the two compositions, we can assert that the formula of the incompletely characterized phase was actually Na6Mg2V4O15.

Experimental top

A single-phase powder sample was obtained by solid-state reaction between Na2CO3, MgO and V2O5. Before weighing, MgO was dried in a dynamic vacuum at 773 K to eliminate water. The starting materials were mixed in an agate mortar, ground under acetone, pressed into pellets and annealed in air at 773 K for 36 h. Single crystals of Na6Mg2V4O15 were obtained by melting the powder sample and then cooling it slowly in? a furnace.

Refinement top

Analysis of the systematic reflection extinctions revealed the following rules: 0kl: k + l = 2n and hk0: h = 2n, which correspond to the two possible space groups Pnma and Pn21a. The former was used for structure solution. The occupancies of atoms Na3 and Na4 appeared to be almost equal and slightly higher than 1/2, and deviations were about 8–10 σ. In this case, the formal oxidation number for vanadium would be higher than +5. Thus, in the final refinement, the occupancies for both Na atoms were fixed to 0.5. Since the structure refinement revealed disorder of atoms V3, Na3 and Na4, additional refinements were performed for space group Pn21a, as well as for other subgroups (down to monoclinic symmetry), to check for possible ordering of the atomic positions. However, lowering the symmetry did not lead to ordering of the cation positions. Moreover, all symmetry-dependent atoms of the Pnma group produced high correlation coefficients (up to 0.99 in the corresponding atomic parameters), indicating orthorhombic symmetry of the unit cell.

Computing details top

Data collection: CAD-4 Manual (Enraf–Nonius, 1988); cell refinement: CAD-4 Manual; data reduction: CSD (Akselrud et al., 1993); program(s) used to solve structure: CSD (Akselrud et al., 1993); program(s) used to refine structure: Jana2000 (Petricek & Dusek, 2000); molecular graphics: ATOMS (Dowty, 1998); software used to prepare material for publication: Jana2000.

Figures top
[Figure 1] Fig. 1. The crystal structure of Na6Mg2V4O15.
[Figure 2] Fig. 2. Two possibilities for V2O7-group orientation. Mg2O6 octahedra and V2O4 tetrahedra are shown as filled polyhedra.
(I) top
Crystal data top
Na6Mg2(VO4)2(V2O7)Reflections used were taken from all octants of reciprocal sphere.
Mr = 630.3Dx = 3.013 (1) Mg m3
Orthorhombic, P_n_m_aMo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2ac 2nCell parameters from 23 reflections
a = 17.080 (3) Åθ = 17.6–20.4°
b = 14.6910 (18) ŵ = 2.97 mm1
c = 5.5356 (7) ÅT = 293 K
V = 1389.0 (3) Å3Arbitrary, colourless
Z = 40.23 × 0.09 × 0.07 mm
F(000) = 1208
Data collection top
κ-geometry
diffractometer
1762 reflections with I > 3σ(I)
Radiation source: X-ray sealed tubeRint = 0.030
Graphite monochromatorθmax = 36.9°, θmin = 2.4°
profile data from θ/1.33θ scansh = 2828
Absorption correction: ψ scan
CAD-4 Manual (Enraf–Nonius, 1988)
k = 244
Tmin = 0.889, Tmax = 1.000l = 09
14190 measured reflections2 standard reflections every 120 min
3546 independent reflections intensity decay: none
Refinement top
Refinement on FWeighting scheme based on measured s.u.'s w = 1/[σ2(F) + 0.0004F2]
R[F2 > 2σ(F2)] = 0.037(Δ/σ)max = 0.0004
wR(F2) = 0.050Δρmax = 1.01 e Å3
S = 1.71Δρmin = 0.79 e Å3
1762 reflectionsExtinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
143 parametersExtinction coefficient: 0.011 (3)
3 restraints
Crystal data top
Na6Mg2(VO4)2(V2O7)V = 1389.0 (3) Å3
Mr = 630.3Z = 4
Orthorhombic, P_n_m_aMo Kα radiation
a = 17.080 (3) ŵ = 2.97 mm1
b = 14.6910 (18) ÅT = 293 K
c = 5.5356 (7) Å0.23 × 0.09 × 0.07 mm
Data collection top
κ-geometry
diffractometer
1762 reflections with I > 3σ(I)
Absorption correction: ψ scan
CAD-4 Manual (Enraf–Nonius, 1988)
Rint = 0.030
Tmin = 0.889, Tmax = 1.0002 standard reflections every 120 min
14190 measured reflections intensity decay: none
3546 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.037143 parameters
wR(F2) = 0.0503 restraints
S = 1.71Δρmax = 1.01 e Å3
1762 reflectionsΔρmin = 0.79 e Å3
Special details top

Refinement. Sfls:_F_calc_weight_full_matrix

Heavy-atom positions were obtained by direct methods (CSD program package). Atomic coordinates for other atoms were found from Fourier and difference Fourier syntheses. Final refinements were carried out with the JANA2000 program package.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
V10.96978 (4)0.250.49547 (13)0.00765 (14)
V20.35833 (3)0.01236 (3)0.51440 (9)0.00839 (11)
V30.68766 (6)0.21378 (6)0.53229 (16)0.0085 (2)0.5
Mg1000.50.0103 (3)
Mg20.32177 (8)0.250.4950 (2)0.0081 (3)
Na10.50142 (9)0.13791 (10)0.5134 (3)0.0242 (4)
Na20.83171 (10)0.12119 (11)0.4773 (3)0.0316 (5)
Na30.16297 (17)0.1369 (2)0.4065 (6)0.0213 (6)0.5
Na40.16104 (17)0.1170 (2)0.5479 (6)0.0213 (6)0.5
O10.8985 (2)0.250.7138 (5)0.0171 (9)
O20.59707 (19)0.250.4602 (6)0.0204 (9)
O30.42109 (19)0.250.2757 (5)0.0159 (8)
O40.21445 (19)0.250.6862 (5)0.0154 (8)
O50.7527 (2)0.250.3249 (5)0.0153 (8)
O60.30663 (14)0.10984 (15)0.4797 (4)0.0186 (6)
O70.02185 (13)0.15062 (16)0.5239 (4)0.0177 (6)
O80.43209 (14)0.01217 (16)0.3088 (4)0.0181 (6)
O90.69933 (15)0.08289 (18)0.5313 (5)0.0260 (7)
O100.89792 (13)0.00843 (16)0.7012 (4)0.0160 (6)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
V10.0076 (3)0.0079 (2)0.0075 (2)00.0005 (2)0
V20.00808 (19)0.00828 (18)0.00882 (19)0.00008 (13)0.00082 (16)0.00018 (16)
V30.0074 (3)0.0092 (4)0.0090 (4)0.0013 (3)0.0001 (3)0.0002 (3)
Mg10.0078 (6)0.0153 (6)0.0077 (5)0.0017 (5)0.0011 (5)0.0001 (6)
Mg20.0082 (6)0.0090 (5)0.0070 (5)00.0009 (5)0
Na10.0252 (7)0.0174 (6)0.0301 (8)0.0043 (6)0.0056 (6)0.0010 (7)
Na20.0380 (9)0.0227 (7)0.0341 (9)0.0098 (7)0.0190 (8)0.0076 (7)
Na30.0179 (8)0.0164 (11)0.0295 (12)0.0062 (8)0.0000 (10)0.0070 (9)
Na40.0179 (8)0.0164 (11)0.0295 (12)0.0062 (8)0.0000 (10)0.0070 (9)
O10.0187 (16)0.0224 (16)0.0103 (13)00.0049 (12)0
O20.0096 (13)0.0297 (18)0.0219 (16)00.0045 (12)0
O30.0135 (14)0.0244 (16)0.0098 (12)00.0039 (11)0
O40.0158 (15)0.0192 (15)0.0112 (12)00.0016 (11)0
O50.0193 (16)0.0142 (13)0.0126 (13)00.0049 (12)0
O60.0175 (10)0.0138 (9)0.0246 (11)0.0038 (8)0.0021 (10)0.0044 (9)
O70.0153 (9)0.0149 (9)0.0230 (11)0.0053 (7)0.0003 (9)0.0006 (10)
O80.0167 (10)0.0236 (11)0.0139 (9)0.0006 (9)0.0044 (8)0.0013 (9)
O90.0204 (11)0.0270 (13)0.0306 (13)0.0062 (10)0.0018 (11)0.0036 (12)
O100.0151 (10)0.0213 (11)0.0115 (9)0.0022 (9)0.0029 (8)0.0005 (8)
Geometric parameters (Å, º) top
V1—O11.715 (3)Na1—O22.338 (3)
V1—O3i1.716 (3)Na1—O32.515 (3)
V1—O7ii1.717 (2)Na1—O7vii2.592 (3)
V1—O7iii1.717 (2)Na1—O82.469 (3)
V2—O61.693 (2)Na1—O8iv2.668 (3)
V2—O81.698 (2)Na2—O12.568 (3)
V2—O9iv1.730 (3)Na2—O3i2.806 (3)
V2—O10v1.714 (2)Na2—O52.473 (3)
V3—V3vi1.0642 (12)Na2—O6i2.572 (3)
V3—O21.684 (3)Na2—O8i2.831 (3)
V3—O4vii1.709 (3)Na2—O92.349 (3)
V3—O51.684 (3)Na2—O102.358 (3)
V3—O91.933 (3)Na3—Na40.836 (5)
Mg1—O72.248 (2)Na3—O2ix2.855 (4)
Mg1—O7viii2.248 (2)Na3—O42.436 (4)
Mg1—O8ix2.072 (2)Na3—O5ix2.599 (4)
Mg1—O8x2.072 (2)Na3—O62.518 (4)
Mg1—O10xi2.074 (2)Na3—O72.505 (4)
Mg1—O10iv2.074 (2)Na3—O9ix2.624 (4)
Mg2—O1v2.078 (4)Na3—O10iv2.448 (4)
Mg2—O32.086 (4)Na4—O42.288 (3)
Mg2—O42.117 (3)Na4—O62.517 (4)
Mg2—O5ix2.127 (3)Na4—O72.432 (4)
Mg2—O62.077 (2)Na4—O8x2.867 (4)
Mg2—O6vi2.077 (2)Na4—O9v2.471 (4)
Na1—O1v2.843 (3)Na4—O10iv2.512 (4)
O1—V1—O3i105.79 (16)O9iv—V2—O10v109.37 (12)
O1—V1—O7ii107.64 (9)V3vi—V3—O271.58 (8)
O1—V1—O7iii107.64 (9)V3vi—V3—O4vii71.86 (7)
O3i—V1—O7ii109.35 (9)V3vi—V3—O571.58 (8)
O3i—V1—O7iii109.35 (9)V3vi—V3—O9174.08 (12)
O7ii—V1—O7iii116.52 (11)O2—V3—O4vii111.32 (15)
O7iii—V1—O7ii116.52 (11)O2—V3—O5110.17 (15)
O6—V2—O8108.20 (12)O2—V3—O9114.10 (10)
O6—V2—O9iv111.76 (12)O4vii—V3—O5110.28 (14)
O6—V2—O10v109.77 (12)O4vii—V3—O9106.54 (10)
O8—V2—O9iv108.86 (12)O5—V3—O9104.14 (10)
O8—V2—O10v108.83 (11)
Symmetry codes: (i) x+1/2, y, z+1/2; (ii) x+1, y, z; (iii) x+1, y+1/2, z; (iv) x+1, y, z+1; (v) x1/2, y, z+3/2; (vi) x, y+1/2, z; (vii) x+1/2, y, z+3/2; (viii) x, y, z+1; (ix) x1/2, y, z+1/2; (x) x+1/2, y, z+1/2; (xi) x1, y, z.

Experimental details

Crystal data
Chemical formulaNa6Mg2(VO4)2(V2O7)
Mr630.3
Crystal system, space groupOrthorhombic, P_n_m_a
Temperature (K)293
a, b, c (Å)17.080 (3), 14.6910 (18), 5.5356 (7)
V3)1389.0 (3)
Z4
Radiation typeMo Kα
µ (mm1)2.97
Crystal size (mm)0.23 × 0.09 × 0.07
Data collection
Diffractometerκ-geometry
diffractometer
Absorption correctionψ scan
CAD-4 Manual (Enraf–Nonius, 1988)
Tmin, Tmax0.889, 1.000
No. of measured, independent and
observed [I > 3σ(I)] reflections
14190, 3546, 1762
Rint0.030
(sin θ/λ)max1)0.846
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.037, 0.050, 1.71
No. of reflections1762
No. of parameters143
No. of restraints3
Δρmax, Δρmin (e Å3)1.01, 0.79

Computer programs: CAD-4 Manual (Enraf–Nonius, 1988), CAD-4 Manual, CSD (Akselrud et al., 1993), Jana2000 (Petricek & Dusek, 2000), ATOMS (Dowty, 1998), Jana2000.

Selected bond lengths (Å) top
V1—O11.715 (3)Na1—O32.515 (3)
V1—O3i1.716 (3)Na1—O7vi2.592 (3)
V1—O7ii1.717 (2)Na1—O82.469 (3)
V2—O61.693 (2)Na1—O8iii2.668 (3)
V2—O81.698 (2)Na2—O12.568 (3)
V2—O9iii1.730 (3)Na2—O52.473 (3)
V2—O10iv1.714 (2)Na2—O6i2.572 (3)
V3—V3v1.0642 (12)Na2—O92.349 (3)
V3—O21.684 (3)Na2—O102.358 (3)
V3—O4vi1.709 (3)Na3—Na40.836 (5)
V3—O51.684 (3)Na3—O42.436 (4)
V3—O91.933 (3)Na3—O5vii2.599 (4)
Mg1—O72.248 (2)Na3—O62.518 (4)
Mg1—O8vii2.072 (2)Na3—O72.505 (4)
Mg1—O10viii2.074 (2)Na3—O9vii2.624 (4)
Mg2—O1iv2.078 (4)Na3—O10iii2.448 (4)
Mg2—O32.086 (4)Na4—O42.288 (3)
Mg2—O42.117 (3)Na4—O62.517 (4)
Mg2—O5vii2.127 (3)Na4—O72.432 (4)
Mg2—O62.077 (2)Na4—O9iv2.471 (4)
Mg2—O6v2.077 (2)Na4—O10iii2.512 (4)
Na1—O22.338 (3)
Symmetry codes: (i) x+1/2, y, z+1/2; (ii) x+1, y, z; (iii) x+1, y, z+1; (iv) x1/2, y, z+3/2; (v) x, y+1/2, z; (vi) x+1/2, y, z+3/2; (vii) x1/2, y, z+1/2; (viii) x1, y, z.
 

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