Two metasilicates with vierer single chains: High-pressure synthesis and structure refinement of Sr-2(VO)(2)Si4O12 (Haradaite) and Sr-2(TiO)(2)Si4O12

Single crystals of Sr-2(VO)(2)Si4O12 could be obtained by high-pressure high-temperature reaction of a SrSiO3/SiO2/V2O5 mixture in a modified Belt-type apparatus. The green crystals are orthorhombic, space group Cmcm, with a = 5.3139(3), b = 14.6281(7), c = 7.0329(4) Angstrom and Z = 2. The structure comprises vierer single chains of SiO4 tetrahedra, the resulting Si4O12 units being connected by VO5 square pyramids. There is one short distance (V - O = 1.61(1) Angstrom) within the square pyramid, giving definite evidence to a (VO)(2+) group with a V=O double bond. The crystal structure of microcrystalline Sr-2(TiO)(2)Si4O12, obtained by high-pressure high-temperature reaction of a SrSiO3/SiO2/TiO2 mixture, was refined by the Rietveld technique. The compound (Cmcm, a = 5.3530(2), b = 14.4864(7), c = 7.0817(3) Angstrom, Z = 2) is isostructural with the vanadium compound.