The layer silicate Cs₂Sn^IVSi₆O₁₅

Single crystals of Cs₂SnSi₆O₁₅, dicaesium tin(IV) hexa-silicate, were serendipitously obtained from a CsCl/NaCl flux at 923 K, starting from mixtures of CaO, SnO and TeO₂ in a closed silica ampoule. The crystal structure of Cs₂SnSi₆O₁₅ is constructed from {Si₆O₁₅}^6- layers extending parallel to (101), and Cs^I cations with a coordination number of eleven as well as isolated [SnO₆] octa-hedra situated between the silicate layers. Each of the nine different SiO₄ tetra-hedra in the silicate layer has a connectedness of Q ³ (three bridging and one terminal O atom), which leads to the formation of five- and eight-membered rings. The same type of silicate layer is found in the crystal structure of the mineral zeravshanite. Comparison with other silicates of the type Cs₂ M ^IVSi₆O₁₅ (M ^IV = Ti, Zr, Th, U) revealed a klassengleiche group-subgroup relationship of index 2 between Cs₂ZrSi₆O₁₅ (Z = 6, space group C2/m) and Cs₂SnSi₆O₁₅ (Z = 12, space group I2/c).