The low-symmetry lanthanum(III) oxotellurate(IV), La10Te12O39

Single crystals of decalanthanum(III) dodecaoxotellurate(IV), La10Te12O39, were obtained by reacting La2O3 and TeO2 in a CsCl flux. Its crystal structure can be viewed as a three-dimensional network of corner- and edge-sharing LaO8 polyhedra with TeIV atoms filling the interstitial sites. The TeIV atoms with their 5s 2 electron lone pairs distort the LaO8 polyhedra through variable Te—O bonds. Among the six unique Te sites, four of them define empty channels extending parallel to the a axis. The formation of these channels is a result of the stereochemically active electron lone pairs on the TeIV atoms. The atomic arrangement of the Te—O units can be understood on the basis of the valence shell electron pair repulsion (VSEPR) model. A certain degree of disorder is observed in the crystal structure. As a result, one of the five different La sites is split into two positions with an occupancy ratio of 0.875 (2):0.125 (2). Also, one of the oxygen sites is split into two positions in a 0.559 (13):0.441 (13) ratio, and one O site is half-occupied. Such disorder was observed in all measured La10Te12O39 crystals.

Single crystals of decalanthanum(III) dodecaoxotellurate(IV), La 10 Te 12 O 39 , were obtained by reacting La 2 O 3 and TeO 2 in a CsCl flux. Its crystal structure can be viewed as a threedimensional network of corner-and edge-sharing LaO 8 polyhedra with Te IV atoms filling the interstitial sites. The Te IV atoms with their 5s 2 electron lone pairs distort the LaO 8 polyhedra through variable Te-O bonds. Among the six unique Te sites, four of them define empty channels extending parallel to the a axis. The formation of these channels is a result of the stereochemically active electron lone pairs on the Te IV atoms. The atomic arrangement of the Te-O units can be understood on the basis of the valence shell electron pair repulsion (VSEPR) model. A certain degree of disorder is observed in the crystal structure. As a result, one of the five different La sites is split into two positions with an occupancy ratio of 0.875 (2):0.125 (2). Also, one of the oxygen sites is split into two positions in a 0.559 (13):0.441 (13) ratio, and one O site is half-occupied. Such disorder was observed in all measured La 10 Te 12 O 39 crystals.  (2000). For standardization of structural data, see: Gelato & Parthé (1987). For the VSEPR model, see: Gillespie (1970 Data collection: X-AREA (Stoe, 2004); cell refinement: X-AREA; data reduction: X-RED32 (Stoe, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Crystal Impact, 2008); software used to prepare material for publication: SHELXL97.

Related literature
compositions, these compounds all contain tellurium atoms in the oxidation state +IV. The remaining two electrons on the Te IV atoms usually form stereo-chemically active but chemically non-bonding electron lone pairs. The local atomic arrangements around these Te IV atoms with electron lone pairs can be understood on basis of the ′Valence Shell Electron Pair Repulsion′ (VSEPR) model (Gillespie, 1970). As the local symmetry of the Te IV sites is reduced by the lone pairs, these compounds usually adopt low-symmetry space groups, e.g. C2/c for Nd 2 Te 4 O 11 , P1 for Ho 2 Te 5 O 13 or P2 1 /c for Dy 2 Te 3 O 9 . The title compound, La 10 Te 12 O 39 , can be considered as another member of the rare-earth(III) oxotellurates(IV) family. Structural similarities can be observed between La 10 Te 12 O 39 and other rare-earth(III) oxotellurates(IV), including the distorted LaO 8 polyhedra and a number of different Te-O structural motifs. Among the six Te atoms in the unit cell, four of them define empty channels parallel to the a-axis, while the other two fill the interstitial positions between the LaO 8 polyhedra (Fig. 1).
If Te-O distances ≤ 2.03 Å (sum of the Te and O covalent radii) are considered as primary Te-O bonds, three TeO 3 units (for Te1, Te4 and Te6) can be identified. Also, a fourth long Te-O interaction is observed for each of these three Te atoms (the fourth oxygen atoms is 2.852 (6) Å from Te1, 2.923 (8) Å from Te4 and 2.802 (6) Å from Te6). Bond valence sum calculations (Brown, 2009) show a minimal contribution from these long interactions; when these interactions are discarded, the bond valence sums for the 3-coordinated Te1, Te4 and Te6 atoms are close to 4.0 valence units (v.u.). The other three Te atoms (Te2, Te3 and Te5) display shorter secondary Te-O interactions (≤ 2.5 Å), which allow them to form TeO 4 units in distorted seesaw configurations. The bond valence sums indicate significant contributions from these interactions. For these TeO 4 units, bond valence sum are 3.9 v.u, 3.9 v.u and 4.4 v.u for Te2, Te3 and Te5, respectively.
Such results are consistent with the assignment of oxidation state +IV for the Te atoms.
The competition between the Te2 and Te5 atoms to form stronger interactions with the bridging oxygen atom (O19) also causes disorder on the oxygen site. As a result, O19 is split into two separated oxygen positions (O191 and O192) to account for the electron density distribution. In addition to the disordered oxygen sites, one of the lanthanum sites (La5) is also described by split positions (La51 and La52). Such disorder could be a consequence of the asymmetric environment in the distorted LaO 8 polyhedron.
The sample mixture was placed in an alumina crucible, which was sealed in a silica tube under vacuum. The sample was supplementary materials sup-2 Acta Cryst. (2013). E69, i36 heated to 1273 K at a rate of 50 K/h. After holding at 1273 K for 72 h, the temperature was then slowly decreased to 1073 K at a rate of 5 K/hour. After annealing at 1073 K for 20 h, the sample was quenched in air. Transparent, colorless, needle-shaped single crystals were obtained by washing away the salt flux with distilled water and ethanol.

Refinement
The structure was standardized by using the STRUCTURE TIDY program (Gelato & Parthé, 1987). Two pairs of split positions, La51/La52 and O191/O192 were modelled to account for the observed electron density distribution. Between each pair of spilt sites, the sum of their occupancies was constrained to 100%, while the isotropic or anisotropic displacement parameters were equalized using the EADP command. The occupancies of each site was extracted from the refinement. Deficiency on the O20 site was observed during the refinement. The occupancy of the O20 site was later assigned to 50% to account for charge balance. The remaining maximum and minimum electron densities (3.71 e -Å -3 and -3.98 e -Å -3 ) are 0.62 Å and 0.24 Å, respectively, from atom Te5.

Figure 1
The crystal structure of La 10 Te 12 O 39 represented with displacement ellipsoids at the 90% probability level.  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq Occ.