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The structure of an already evidenced but still uncharacterized GeTe2O6 phase consists of isolated GeO6 octa­hedra connected via isolated TeO3 units. The germanium cations occupy a site with \overline{1} symmetry. The Te and O atoms are in general positions of the P21/n space group. This structure corresponds to a new type of tetra­valent tellurate and is different from other AB2X6 structures in which the B cation presents a stereochemically active electronic lone pair. It derives from the pseudo-hexa­gonal MI2O6 (M = Mg, Mn, Co and Fe) type by a strong monoclinic distortion caused by the much smaller size of Ge4+ compared with the divalent M cations.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270109010488/sq3186sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270109010488/sq3186Isup2.hkl
Contains datablock I

Comment top

Germanium oxide has been used as a building element to form a number of open frameworks with novel topologies. The structure of the germanate framework can be formed by GeO4 (tetrahedra), GeO6 (octahedra) and sometimes GeO5 (square pyramid or trigonal bipyramid) (Liu et al., 2008) polyhedra. Meanwhile, our laboratory has systematically developed the investigation of tellurium(IV) compounds for their potential nonlinear optical properties (Laval et al., 2008). We have attempted to combine the building capability of Ge(IV) oxide with Te(IV) oxide in an effort to obtain novel germanium oxyfluorotellurates. In this paper, we report the structure of the oxide GeTe2O6, which was inadvertently obtained in one of our reactions. The powder X-ray diffraction pattern has been reported (PDF No. 00-051-0288; Gospodinov, 1999); however, the pattern calculated from the present crystal structure differs from the reference pattern, suggesting that the PDF file does not correspond to a pure phase or to the same polymorph. Bond valence calculations (Brown, 1981) confirm that the studied crystal corresponds to an oxide and not to an oxyfluoride as expected (Table 2).

In this structure, the Ge atom is sixfold coordinated, occupying the center of an almost regular octahedron (Fig. 1). The Ge—-O distances are essentially the same within experimental uncertainty (Table 1) and are typical for germanates (Monge et al., 2000; Cascales et al., 1998; Xu et al., 2004).

The configuration of the Te atom is the same as that in MTeO3F (M = Fe, Ga and Cr; Laval et al., 2008), i.e. strongly bonded to three O atoms (O1, O2 and O3; Table 1) at the center of a tetrahedron whose fourth corner corresponds to the direction of the stereochemically active lone pair E (Fig. 2). Three additional weak Te—O bonds can be added to the coordination environment of the Te atom. In that case, the corresponding polyhedron can be roughly described as a distorted octahedron. The lone pair E points towards the face formed by atoms O1i, O1ii and O3iii [symmetry codes: (i) - x + 1, - y, - z + 1; (ii) x + 1/2, - y + 1/2, z + 1/2; (iii) - x + 1/2, y - 1/2, - z + 3/2].

The GeTe2O6 structure is based on the association, by corner-sharing, of GeO6 octahedra and TeO3 trigonal pyramids. Each Te atom is bonded to three different Ge atoms via oxygen vertices, and conversely each GeO6 octahedron is linked via six TeO3 bridges to ten other GeO6 octahedra. The projections onto the three main planes of the almost orthorhombic structure (Fig. 3) show that the GeO6 octahedra are tilted along two directions ([010] and [010]) and form layers perpendicular to [001], each one alternating with a wavy layer of tellurium. The three-dimensional framework of Te and Ge cations derives from a hexagonal packing, but with great distortion (a/b = 0.749 for the orthorhombic cell, instead of 0.866 associated with an ideal hexagonal cell).

If the weak Te—O bonds are considered, Te4O20 units are formed. These units are connected via O1 vertices to form infinite rows along [010] with cavities of irregular cross section (Fig. 4a). The GeO6 octahedra ensure the connection of these rows (Fig. 4b).

The MxTeyOz tellurates(IV) present many M/Te compositions, from 5/1 (Mo5TeO16) to 1/6 (ZnTe6O13). However, the main M/Te ratios for di-, tri-, tetra-, penta- or hexavalent metal oxides are 1/1 [e.g. CoTeO3, VTeO4, Ta2(V2)Te2O9], 3/2 [e.g. Ni2Te3O8, Fe2(or In2)Te3O9, Nb2Te3O11], 1/2 [e.g. MgTe2O5, Cr2(or Ln2)Te4O11, Th(or Ce or Pu)Te2O6, Nb2Te4O13, MoTe2O7], 5/2 [e.g. Sc2(or Lu2)Te5O13], 1/3 [e.g. Zr(or Sn or Hf)Te3O8], 1/5 (e.g. PbTe5O11) and 1/6 (e.g. ZnTe6O13) (FIZ/NIST, 2008).

These phases present a very rich crystallochemistry, with structures generally consisting of more or less complex associations (groups, chains or layers) of MO6 octahedra and of TeO3 or TeO4 polyhedra. GeTe2O6 is a new structure type for tetravalent tellurates and is simpler than most of the phases noted above. In fact, few tellurates(IV) contain isolated MO6 octahedra, except ZnTe6O13. The other MTe2O6 tellurates adopt a structure type deriving from fluorite, suitable for tetravalent cations of greater size, such as Ce, Pu and Th (Lopez et al., 1991; Krishnan et al., 2000).

The new GeTe2O6 type is structurally closer to the MI2O6 series of iodates(V) with divalent cations Mg, Mn, Co, Ni and Zn (Phanon et al., 2006). These MI2O6 compounds are isostructural and crystallize in the monoclinic system (space group P21), but are very close to hexagonal symmetry (β 120°). The general organization of the structures is similar, but GeTe2O6 is much more distant from the ideal hexagonal symmetry than the MI2O6 series, as discussed above and as is easily observed on comparing Figs. 3(a) and 5. This higher distortion likely results from the lower size of the Ge4+ cation (R = 0.53 Å) compared with the size of the M cations of the MI2O6 series (about 0.65–0.75 Å). This prevents the TeO3 polyhedra from adopting a nearly regular hexagonal framework and causes a tilting of the GeO6 octahedra.

Related literature top

For related literature, see: Bruker (2001); Cascales et al. (1998); FIZ/NIST (2008); Laval et al. (2008); Liu et al. (2008); Monge et al. (2000); Phanon et al. (2006); Sheldrick (2008); Xu et al. (2004).

Experimental top

Small single crystals of GeTe2O6 were accidentally obtained in experiments initially intended to synthesize new oxyfluorotellurates(IV). TeO2 was prepared in the laboratory by decomposition at 823 K under flowing oxygen of commercial H6TeO6 (Aldrich, 99.9%) and GeO2 was a commercial product (Aldrich, 99.9%). An equimolar mixture of GeO2 and TeO2 was dissolved in hydrofluoric acid (40%) in a Teflon beaker and heated at 453 K. Then, after slow evaporation, the product was crushed and heated at 673 K in a platinum set [test?] tube for 48 h. Small colourless tablets of GeTe2O6, air stable and suitable for a structural study, were obtained instead of the expected germanium oxyfluorotellurate(IV).

Refinement top

The integrated intensities were corrected for absorption effects by using a multi-scan method (SADABS; Bruker, 2001). Structure solution by direct methods in the P21/n space group, followed by refinement of atomic coordinates and anisotropic thermal parameters, were performed using successively the SHELXS97 and the SHELXL97 programs (Sheldrick, 2008).

Computing details top

Data collection: KappaCCD Server Software (Nonius, 1998); cell refinement: DIRAX/LSQ (Duisenberg, 1992); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. : The coordination polyhedron of Ge1 in the GeTe2O6 structure. [Symmetry codes: (ii) x + 1/2, -y + 1/2, z + 1/2; (iv) x - 1/2, -y + 1/2, z + 1/2; (v) -x + 1, -y + 1, -z + 2; (vi) -x + 3/2, y + 1/2, -z + 3/2; (vii) -x + 1/2, y + 1/2, -z + 3/2.]
[Figure 2] Fig. 2. : The anionic polyhedron around the Te4+ cation in the GeTe2O6 structure. The arrow indicates the direction in which the lone pair E points. Broken lines represent weak Te1—O bonds. [Symmetry codes: (i) -x + 1, - y, -z + 1; (ii) x + 1/2, -y + 1/2, z + 1/2; (iii) -x + 1/2, y - 1/2, -z + 3/2.]
[Figure 3] Fig. 3. : Projections onto (a) the xy, (b) the xz and (c) the yz planes, showing the GeO6 octahedra and their connection via TeO3 polyhedra.
[Figure 4] Fig. 4. : (a) A perspective view showing the double chains of TeO6 distorted octahedra. (b) A projection onto yz showing the global structure of GeTe2O6 with the six-membered-ring channels.
[Figure 5] Fig. 5. : A projection onto the xz plane of the MgI2O6 structure for comparison with GeTe2O6 (Fig. 3a).
germanium tellurate(IV) top
Crystal data top
GeTe2O6F(000) = 368
Mr = 423.79Dx = 5.281 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P2ynCell parameters from 2864 reflections
a = 5.2201 (8) Åθ = 4.9–30.0°
b = 6.9730 (13) ŵ = 16.43 mm1
c = 7.3252 (15) ÅT = 293 K
β = 91.66 (2)°Tablet, colourless
V = 266.52 (8) Å30.02 × 0.01 × 0.003 mm
Z = 2
Data collection top
Nonius KappaCCD
diffractometer
771 independent reflections
Radiation source: fine-focus sealed tube551 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.105
Detector resolution: 9 pixels mm-1θmax = 30.0°, θmin = 4.9°
CCD scansh = 77
Absorption correction: multi-scan
(SADABS; Bruker 2004)
k = 99
Tmin = 0.735, Tmax = 0.952l = 1010
5004 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.043 w = 1/[σ2(Fo2) + (0.0161P)2 + 0.228P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.055(Δ/σ)max < 0.001
S = 1.05Δρmax = 2.24 e Å3
771 reflectionsΔρmin = 1.72 e Å3
44 parametersExtinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.0024 (6)
Crystal data top
GeTe2O6V = 266.52 (8) Å3
Mr = 423.79Z = 2
Monoclinic, P21/nMo Kα radiation
a = 5.2201 (8) ŵ = 16.43 mm1
b = 6.9730 (13) ÅT = 293 K
c = 7.3252 (15) Å0.02 × 0.01 × 0.003 mm
β = 91.66 (2)°
Data collection top
Nonius KappaCCD
diffractometer
771 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker 2004)
551 reflections with I > 2σ(I)
Tmin = 0.735, Tmax = 0.952Rint = 0.105
5004 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.04344 parameters
wR(F2) = 0.0550 restraints
S = 1.05Δρmax = 2.24 e Å3
771 reflectionsΔρmin = 1.72 e Å3
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Te10.52166 (8)0.15761 (7)0.71712 (7)0.00782 (16)
Ge10.50000.50001.00000.0069 (3)
O10.3012 (8)0.1473 (7)0.5059 (7)0.0104 (11)
O20.8158 (9)0.2115 (7)0.5856 (7)0.0101 (12)
O30.4440 (9)0.4156 (7)0.7598 (7)0.0101 (12)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Te10.0068 (2)0.0082 (2)0.0085 (3)0.0001 (2)0.00028 (16)0.0003 (3)
Ge10.0053 (5)0.0075 (6)0.0078 (6)0.0004 (5)0.0008 (4)0.0000 (5)
O10.008 (2)0.011 (3)0.012 (3)0.002 (2)0.002 (2)0.001 (3)
O20.009 (2)0.007 (3)0.014 (3)0.003 (2)0.002 (2)0.003 (2)
O30.012 (3)0.012 (3)0.007 (3)0.001 (2)0.002 (2)0.002 (2)
Geometric parameters (Å, º) top
Te1—O31.873 (5)Ge1—O3iv1.870 (5)
Te1—O21.874 (5)Ge1—O1v1.877 (4)
Te1—O11.903 (5)Ge1—O1ii1.877 (4)
Te1—O1i2.852 (5)Ge1—O2vi1.878 (5)
Te1—O1ii2.877 (5)Ge1—O2vii1.878 (5)
Te1—O3iii2.968 (5)O1—Ge1iii1.877 (4)
Ge1—O31.870 (5)O2—Ge1viii1.878 (5)
O3—Te1—O294.5 (2)O3iv—Ge1—O1v87.6 (2)
O3—Te1—O192.5 (2)O3—Ge1—O1ii87.6 (2)
O2—Te1—O194.3 (2)O3iv—Ge1—O1ii92.4 (2)
O3—Te1—O1i152.54 (19)O1v—Ge1—O1ii180.000 (1)
O2—Te1—O1i64.59 (17)O3—Ge1—O2vi90.0 (2)
O1—Te1—O1i72.81 (19)O3iv—Ge1—O2vi90.0 (2)
O3—Te1—O1ii62.00 (16)O1v—Ge1—O2vi90.0 (2)
O2—Te1—O1ii82.96 (18)O1ii—Ge1—O2vi90.0 (2)
O1—Te1—O1ii153.87 (8)O3—Ge1—O2vii90.0 (2)
O1i—Te1—O1ii127.44 (9)O3iv—Ge1—O2vii90.0 (2)
O3—Te1—O3iii110.80 (9)O1v—Ge1—O2vii90.0 (2)
O2—Te1—O3iii145.88 (18)O1ii—Ge1—O2vii90.0 (2)
O1—Te1—O3iii63.11 (17)O2vi—Ge1—O2vii180.000 (1)
O1i—Te1—O3iii83.60 (13)Ge1iii—O1—Te1121.7 (3)
O1ii—Te1—O3iii128.74 (14)Te1—O2—Ge1viii116.7 (2)
O3—Ge1—O3iv180.000 (2)Ge1—O3—Te1115.5 (2)
O3—Ge1—O1v92.4 (2)
Symmetry codes: (i) x+1, y, z+1; (ii) x+1/2, y+1/2, z+1/2; (iii) x+1/2, y1/2, z+3/2; (iv) x+1, y+1, z+2; (v) x+1/2, y+1/2, z+3/2; (vi) x+3/2, y+1/2, z+3/2; (vii) x1/2, y+1/2, z+1/2; (viii) x+3/2, y1/2, z+3/2.

Experimental details

Crystal data
Chemical formulaGeTe2O6
Mr423.79
Crystal system, space groupMonoclinic, P21/n
Temperature (K)293
a, b, c (Å)5.2201 (8), 6.9730 (13), 7.3252 (15)
β (°) 91.66 (2)
V3)266.52 (8)
Z2
Radiation typeMo Kα
µ (mm1)16.43
Crystal size (mm)0.02 × 0.01 × 0.003
Data collection
DiffractometerNonius KappaCCD
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker 2004)
Tmin, Tmax0.735, 0.952
No. of measured, independent and
observed [I > 2σ(I)] reflections
5004, 771, 551
Rint0.105
(sin θ/λ)max1)0.704
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.043, 0.055, 1.05
No. of reflections771
No. of parameters44
Δρmax, Δρmin (e Å3)2.24, 1.72

Computer programs: KappaCCD Server Software (Nonius, 1998), DIRAX/LSQ (Duisenberg, 1992), EVALCCD (Duisenberg et al., 2003), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 1999).

Selected bond lengths (Å) top
Te1—O31.873 (5)Te1—O3iii2.968 (5)
Te1—O21.874 (5)Ge1—O31.870 (5)
Te1—O11.903 (5)Ge1—O1ii1.877 (4)
Te1—O1i2.852 (5)Ge1—O2iv1.878 (5)
Te1—O1ii2.877 (5)
Symmetry codes: (i) x+1, y, z+1; (ii) x+1/2, y+1/2, z+1/2; (iii) x+1/2, y1/2, z+3/2; (iv) x1/2, y+1/2, z+1/2.
Table 2. Bond valences for GeTe2O6 top
AtomsTe1Ge1Vij
O11.223/0.094/0.0882 × 0.7042.11
O21.3212 × 0.7042.03
O31.326/0.0692 × 0.7192.11
Vij4.124.25
 

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