Ca2Te3O8, a new phase in the CaO–TeO2 system

The crystal structure of Ca2Te3O8 is isotypic with Pb2Te3O8.

Single crystals of dicalcium octaoxidotritellurate(IV), Ca 2 Te 3 O 8 , were obtained from a CsCl/NaCl melt with CaO and TeO 2 as educts in the molar ratio of 1:2. Ca 2 Te 3 O 8 crystallizes isotypically with Pb 3 Te 2 O 8 and is comprised of two unique Ca, four Te and eight O sites. One calcium cation has eight and the other nine coordination partners. Both coordination polyhedra are considerably distorted. Two kinds of oxotellurate(IV) anions with the same formula [Te 3 O 8 ] 4À are present. One is an infinite zigzag chain anion consisting of pairs of [TeO 4 ] bisphenoids linked to a trigonal-pyramidal [TeO 3 ] group with a connectivity of [(TeO 1/1 O 2/2 )(TeO 2/1 O 2/2 ) 2 ] n , while the other is a finite anion made up of one central [TeO 4 ] bisphenoid linked to two [TeO 3 ] trigonal pyramids and has a connectivity of [(TeO 2/1 O 1/2 ) 2 (TeO 2/2 O 2/1 )]. In the crystal, the anions are organized in layers extending parallel to (100). Adjacent layers are held together by the calcium cations to define a three-dimensional framework structure.

Chemical context
A partial phase diagram for the pseudo-binary system CaO-TeO 2 has been determined for the composition range 50-100 mol% TeO 2 to contain the 1:1 phase CaTeO 3 and the 1:2 phase CaTe 2 O 5 (Mishra et al., 1998). Another phase not reported during the original study of Mishra et al. (1998) is the 4:5 phase Ca 4 Te 5 O 14 , for which full structural details were determined for the normal-pressure and high-pressure forms (Weil, 2004;Weil et al., 2016). For compositions CaTeO 3 and CaTe 2 O 5 , polymorphism was reported on the basis of differential thermal analysis and temperature-dependent X-ray diffracion (Mishra et al., 1998;Tripathi et al., 2001), however, without structural details of the corresponding phases. Whereas crystal structure determinations were subsequently performed for four polymorphic forms of CaTeO 3 (Stö ger et al., 2009;Poupon et al., 2015), our present knowledge of the CaTe 2 O 5 structures is restricted to only one form (Weil & Stö ger, 2008;Barrier et al., 2009) that is not related to the mica-like CaTe 2 O 5 phase reported nearly 50 years ago (Redman et al., 1970). In an attempt to grow single crystals of the latter from a salt melt at comparatively low temperatures, a heretofore unknown phase in the CaO-TeO 2 system was obtained, viz. the 2:3 phase Ca 2 Te 3 O 8 .
In this article, preparation conditions, crystal structure and the relation to the isotypic lead(II) analogue Pb 2 Te 3 O 8 (Champarnaud-Mesjard et al., 2001) are reported.
All four Te atoms have an oxidation state of +IV and can be divided into two pairs with the most commonly observed three-coordination in the form of a trigonal pyramid (Te2 and Te4) and four-coordination in the form of a bisphenoid (Te1 and Te3). The Te-O bond lengths within the [TeO 3 ] trigonal pyramids are only slightly spread, ranging from 1.8522 (12) to 1.8994 (11) Å . The two [TeO 4 ] bisphenoids are characterised by two short bonds of < 2 Å and two longer bonds of > 2 Å , with the maximum at 2.3222 (12) Å for Te3. All Te-O bond lengths (Table 1) are in characteristic ranges for oxotellurates(IV) with three-and four-coordinate tellurium, as reviewed recently by Christy et al. (2016).
In the crystal, the two types of [Te 3 O 8 ] 4À anions are arranged in layers parallel to (100). Approximately at x ' 1/4 and 3/4, the calcium cations link adjacent layers into the threedimensional framework (Fig. 3). Ca 2 Te 3 O 8 is isotypic with Pb 2 Te 3 O 8 (Champarnaud-Mesjard et al., 2001), but not with its higher alkaline earth homologue Sr 2 Te 3 O 8 , which is reported to have a different orthorhombic cell, with details of the structure not known (Elerman & Koçak, 1986 (Table 1) reveals nearly identical values for the individual oxotellurate(IV) units, but differences up to 0.6 Å for the metal-oxygen polyhedra. On one hand, this behaviour is ascribed to the different ionic radii for eight-coordinate Ca II and Pb II of 1.12 and 1.29 Å , respectively (Shannon, 1976), and, on the other hand, to the stereochemical activity (Galy et al., 1975) of the 6s 2 free-electron lone pair located at Pb II that is responsible for the formation of off-centred lead-oxygen polyhedra with either holo-or hemidirected oxygen ligands (Shimoni-Livny et al., 1998).
For a quantitative structural comparison of the isotypic M 2 Te 3 O 8 (M = Ca, Pb) structures, the program compstru (de la Flor et al., 2016), available at the Bilbao Crystallographic Server (Aroyo et al., 2006), was used. The degree of lattice distortion is 0.0205, the maximum distance between the atomic positions of paired atoms is 0.403 Å for pair O8, the arithmetic mean of all distances is 0.195 Å and the measure of similarity is 0.05.

Synthesis and crystallization
Crystals of Ca 2 Te 3 O 8 were obtained as one of the products from a flux synthesis using a CsCl/NaCl salt mixture (molar ratio 0.65/0.35). To 1.5 g of the salt mixture were added CaO (0.075 g; freshly prepared by heating CaCO 3 at 1473 K for 1 d) and TeO 2 (0.425 g) according to a molar ratio of 1:2. The reaction mixture was placed in a silica ampoule that was subsequently evacuated and sealed. The ampoule was placed vertically in a furnace and heated from room temperature within 3 h to 793 K, kept at that temperature for 90 h and cooled within 10 h to room temperature. The silica ampoule was broken and the solidified melt leached out with water for two h. The colourless product was filtered off, washed with water and was dried in a stream of air. The title compound was present in the form of a few crystals that were distinguishable from the other crystals due to their characteristic square form (maximum edge length 1.5 mm). Other phases identified by single-crystal X-ray diffraction measurements of selected crystals and by powder X-ray diffraction measurements of the bulk were CaTe 2 O 5 in the mica-like modification reported by Redman et al. (1970) as the main phase (tiny colourless plates) and Ca 4 Te 5 O 14 (small colourless pinacoids; Weil, 2004).

Dicalcium octaoxidotritellurate(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )