Single crystals of Sr2(VO)2Si4O12 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) Å 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) Å) within the square pyramid, giving

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... e evidence to a (VO)2+ group with a V = O double bond. The crystal structure of microcrystalline Sr2(TiO)2Si4O12, obtained by high-pressure hightemperature 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) Å, Z = 2) is isostructural with the vanadium compound.