Single crystals of SnTe3O8 in the millimetre range grown by chemical vapor transport reactions

SnTe3O8 crystallizes isotypically with other members of the series M IVTeIV 3O8 (M = Ti, Zr, Hf). It comprises [SnO6] octahedra and [TeO4] bisphenoids as the principal structural building blocks.


Chemical context
The crystal chemistry of oxidotellurates(IV) is dominated by the presence of the 5s 2 electron lone pair that, in the majority of cases, is stereochemically active, thus enabling one-sided coordination spheres around the Te IV atom (Christy et al., 2016). This peculiar building block often results in compounds with non-centrosymmetric structures or structures with polar directions exhibiting interesting physical properties (Ra et al., 2003;Kim et al., 2014). In this context, the microwave dielectric properties of M IV Te 3 O 8 (M = Sn, Zr) ceramics were investigated some time ago (Subodh & Sebastian, 2008).
The crystal structure of the isotypic series M IV Te 3 O 8 was originally determined for M = Ti from a single crystal in space group Ia3 using photographic Weissenberg X-ray data, whereas for M = Sn, Zr and Hf, the crystal structures were refined from powder X-ray data (Meunier & Galy, 1971). In subsequent studies, crystal-structure refinements on the basis of single-crystal X-ray data were reported for the mineral winstanleyite with composition (Ti 0.96 Fe 0.04 )Te 3 O 8 (Bindi & Cipriani, 2003), and for the synthetic compound ZrTe 3 O 8 (Noguera et al., 2003;Lu et al., 2019). A powder X-ray study of the solid solution Sn 0.59 Ti 0.41 Te 3 O 8 crystallizing in the M IV Te 3 O 8 structure type has also been reported (Ben Aribia et al., 2008).
Single-crystal growth of oxidotellurates(IV) can be accomplished through various crystallization methods including, for example, experiments under hydrothermal conditions (Weil et al., 2018), cooling from the melt (Stö ger et al., 2009), from salt melts as fluxing agents (Weil, 2019), or from chemical vapor transport reactions (Missen et al., 2020). The latter method (Binnewies et al., 2012) is particularly suitable for growing large crystals of high quality and was the ISSN 2056-9890 method of choice for crystal growth of SnTe 3 O 8 for which a more precise and accurate structure refinement appeared to be desirable.

Structural commentary
The asymmetric unit of SnTe 3 O 8 comprises one Sn IV atom, one Te IV atom, and two oxide anions, residing on sites 8a (site symmetry .3.), 24d (2..), 48e (1) and 16c (.3.), respectively. The tin atom is in an almost regular octahedral coordination by oxygen, with six equal Sn1-O1 distances, all trans angles equal to 180 , and cis angles ranging from 86.09 (4) to 93.91 (4) . The Te1 site is coordinated by four O atoms in pairs of shorter (O1) and longer distances (O2) ( Table 1). The resulting [TeO 4 ] coordination polyhedron is a distorted bisphenoid. Considering the 5s 2 electron lone pair at the Te IV atom, the corresponding [ÉTeO 4 ] polyhedron has a shape intermediate between a square pyramid and a trigonal bipyramid with the non-bonding electron pair occupying an equatorial position (Fig. 1). The geometry index 5 of the [ÉTeO 4 ] polyhedron is 0.471 ( 5 = 0 for an ideal square pyramid and 5 = 1 for an ideal trigonal bipyramid; Addison et al., 1984). The position of the electron lone pair was calculated with the LPLoc software (Hamani et al., 2020), with resulting fractional coordinates of x = 0.28655, y = 0, z = 1/4. The radius of the electron lone pair was calculated to be 1.07 Å with a distance of 0.90 Å from the Te1 position. The coordination numbers of the oxide anions are two and three: O1 coord-inates to Sn1 and Te1 at the shorter of the two Te1-O distances whereas O2 coordinates to three Te1 atoms at the longer of the two Te1-O distances.
In the crystal structure of SnTe 3 O 8 , the [SnO 6 ] octahedra are isolated from each other and arranged in rows running parallel to [100]. Each of the [TeO 4 ] bisphenoids shares corners (O2) with other [TeO 4 ] bisphenoids to form a threedimensional oxidotellurate(IV) framework. The [SnO 6 ] octahedra are situated in the voids of this framework, thereby sharing each of the six corners with an individual [TeO 4 ] bisphenoid. The crystal structure of SnTe 3 O 8 is depicted in Fig. 2.
The unit-cell parameter a from the previous powder X-ray study, 11.144 (3) Table 1 Selected geometric parameters (Å , ).

Figure 2
The crystal structure of SnTe 3 O 8 in polyhedral representation, showing a projection along [100]. Displacement ellipsoids are as in Fig. 1

Figure 1
The coordination environment around Te1. Displacement ellipsoids are drawn at the 90% probability level; the electron lone pair is given as a green sphere of arbitrary radius. [Symmetry codes: (v) -z + 1 2 , x-1/2, y; (vii) Àz + 1 2 , Àx + 1 2 , Ày + 1 2 ; (viii) x, Ày, Àz + 1 2 .] Te1-O2 0 = 156.8 (Meunier & Galy, 1971) agree with the present single-crystal study (Table 1), but with lower precision and accuracy. In comparison with the previous model based on powder X-ray data, the values of the bond-valence sums (Brown, 2002) (Meunier & Galy, 1971;Wells, 1975). The unit-cell parameter a of cubic TiTe 3 O 8 is $2a of cubic CaF 2 , whereby the ordered distribution of the cationic sites leads to a doubling of the unit cell and also to a considerable distortion of the respective coordination environments. The original cubic coordination around the Ca II cation in the fluorite structure is changed to an octahedral coordination of Sn IV and a fourfold coordination of Te IV in the superstructure of the M IV Te 3 O 8 compounds. Note that there are two additional O atoms at a distance of 3.2446 (19) Å around the M IV site and two pairs of additional O atoms at a distance of 2.9076 (12) and 3.3957 (13) Å around the Te1 site in SnTe 3 O 8 , completing an eightfold coordination in each case. Correspondingly, each of the two O sites has a fourfold coordination in case the much longer distances are counted.
A quantitative structural comparison of the M IV Te 3 O 8 structures where single crystal data are available (M = Ti, Zr, Sn) was undertaken with the program compstru (de la Flor et al., 2016) available at the Bilbao Crystallographic Server (Aroyo et al., 2006). Table 2 lists the degree of lattice distortion (S), the maximum distance between the atomic positions of paired atoms (|u|), the arithmetic mean of all distances, and the measure of similarity (Á) relative to SnTe 3 O 8 as the reference structure. All these values show a very high simi-larity between the crystal structures in the isotypic M IV Te 3 O 8 series.

Synthesis and crystallization
Reagent-grade chemicals were used without further purification. SnO 2 (71 mg, 0.47 mmol) and TeO 2 (225 mg, 1.40 mmol) were thoroughly mixed in the molar ratio 1:3 and placed in a silica tube to which 50 mg of TeCl 4 were added as the transport agent. The silica ampoule was then evacuated and torchsealed, placed in a two-zone furnace using a temperature gradient 973 K (source) ! 873 K (sink) for three days. Cubic, canary-yellow crystals had formed in the millimetre size range in the colder sink region as the only product (Fig. 3). Powder X-ray diffraction of the remaining material in the source region revealed SnTe 3 O 8 as the main phase and SnO 2 as a side phase. For the single-crystal diffraction study, a fragment was broken from a larger crystal.

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