The layer silicate Cs2SnIVSi6O15

The crystal structure of Cs2SnSi6O15 shows a klassengleiche group–subgroup relationship of index 2 with Cs2ZrSi6O15.


Chemical context
Calcium oxotellurate(IV), CaTeO 3 , is known to exist in various polymorphic forms that can be obtained either through solid-state reactions (Trö mel & Ziethen-Reichenach, 1970;Stö ger et al., 2009) or under hydrothermal conditions and subsequent dehydration (Poupon et al., 2015). Some of the CaTeO 3 polymorphs have a non-centrosymmetric crystal structure and show ferroelectric behaviour (Rai et al., 2002) or a second harmonic generation (SHG) effect (Poupon et al., 2015). These features are thought to be related to the presence of the 5s 2 electron lone pair at the Te IV atom. In a current study it was attempted to incorporate Sn II into CaTeO 3 under formation of solid solutions (Ca 1-x Sn x )TeO 3 in order to investigate whether the 5s 2 electron lone pair at the Sn II atom has any influence on the ferroelectric or SHG characteristics. For that purpose, a flux medium consisting of a eutectic CsCl/NaCl mixture with a melting point of 766 K (Ż emcżużny & Rambach, 1909) was chosen as reaction medium in a closed silica ampoule. In comparison with conventional ceramic routes, this allows the lowering of the reaction temperatures by a simultaneous increase of the diffusion pathways. However, neither the intended (Ca 1-x Sn x )TeO 3 solid solutions nor other calcium oxotellurates with a partial replacement of Ca II by Sn II could be prepared this way. One of the side products of this reaction was Cs 2 Sn IV Si 6 O 15 , which had formed through attack of the silica glass ampoule by the molten salt mixture and concomitant oxidation of Sn II to Sn IV . Its crystal structure along with a structural comparison with other silicates is given here. ISSN 2056-9890

Structural commentary
The asymmetric unit of Cs 2 SnSi 6 O 15 comprises three Cs, two Sn, nine Si and twenty-three O sites. Except for one Sn site (Sn2) and one O site (O23), which are located on Wyckoff positions 4b (site symmetry 1) and 4e (site symmetry 2), respectively, all atoms are on general positions. The crystal structure of Cs 2 SnSi 6 O 15 can be described as being built up from silicate layers extending parallel to (101). The silicate layers are linked by caesium cations and isolated [SnO 6 ] octahedra situated between adjacent silicate layers ( Fig. 1).
Each of the three caesium cations exhibits a coordination number of 11, with Cs-O bond lengths ranging from 2.951 (3) to 3.748 (3)

Figure 1
The

Synthesis and crystallization
1.2 mmol of CaO (0.067 g), 0.13 mmol SnO (0.018 g) and 1.3 mmol of TeO 2 (0.207 g) were intimately mixed with 1 g of an NaCl (35 mol%):CsCl (65 mol%) mixture and filled in a silica ampoule that was subsequently evacuated and torchsealed. The ampoule was then heated at 923 K for 2 d before the power of the furnace was switched off. The silica ampoule showed a severe attack from the halide flux but was still intact. After washing the recrystallized flux with several portions of warm water and drying the remaining solid in air, a few lathshaped crystals of the title compound could be isolated under a polarizing microscope.  (Weil, 2004) as products. Powder X-ray diffraction of the bulk showed the reflections of these phases together with those of SnO 2 and also some reflections of non-assignable phase(s).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. For better comparison of Cs 2 SnSi 6 O 15 with the crystal structure of Cs 2 ZrSi 6 O 15 , the unconventional setting I2/c of space group type C2/c (No. 15) was chosen, so that unit-cell parameters a, b, c and of the Sncontaining phase correspond to a, b, 2c and of the Zrcontaining phase (Jolicart et al., 1996; Table 1). The Cs3 atom in Cs 2 SnSi 6 O 15 was found to be disordered over two sites in a 0.934 (5):0.066 ratio and was refined with common displacement parameters for the two sites (A and B).

Data collection
Bruker APEXII CCD diffractometer ω-and φ-scans Absorption correction: multi-scan (SADABS; Krause et al., 2015). T min = 0.539, T max = 0.747 51032 measured reflections 8282 independent reflections 5013 reflections with I > 2σ(I) 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.