Crystal structure of a new vanadate variety in the lomonosovite group: Na5Ti2O2[Si2O7](VO4)

The title compound has been prepared by a flux crystallisation method and i ts crystal structure was determined by single crystal X-ray diffraction: sp ace group P, a = 5.309(1), b = 7.133(1), c = 14.746(2) Angstrom, alpha = 99 .05(1), beta = 95.97(1), y= 90.08(1)degrees, wR(2) = 0.073, R = 0.028. The structure may be described as built by seidozerite modules of composition N a2Ti2O2Si2O7 - brucite-type layers of [TiO6] and [NaO6] octahedra embedded between layers of [TiO6] octahedra, [Si2O7] groups and [NaO8] polyhedra. Th ese almost centrosymmetrical triple-layers alternate along the c-axis with polar double-layer-modules of composition Na3VO4 formed by isolated [VO4](3 -) anions and six- and four-coordinate Na cations. The crystal structure i s discussed in context with minerals of the lomonosovite group. The thermal decomposition behaviour suggests a decay to the single modular components Na2Ti2O2Si2O7 and Na3VO4. (C) 2000 Editions scientifiques et medicales Else vier SAS. All rights reserved.