Synthesis and crystal structure of Na4[(TiO)4(SiO3)3]•6H2O, a rhombohedrally distorted sodium titanium silicate pharmacosiderite analogue

The synthesis and crystal structure of Na-4[(TiO)(4)(SiO4)(3)] . 6H(2)O, a new hydrated sodium titanium silicate with the rhombohedrally distorted pharmacosiderite structure type, are reported. Na-4[(TiO)(4)(SiO4)(3)] . 6H(2)O was synthesized as a powder by the low temperature hydrothermal crystallization of an alkaline titanium silicate gel prepared via a novel peroxide route. Its structure was determined and refined by the Rietveld method using X-ray powder data. The three-dimensional framework of Na-4[(TiO)(4)(SiO4)(3)] . 6H(2)O is built up from clusters of four edge-sharing TiO6 octahedra, connected by tetrahedrally coordinated silicon atoms (isolated SiO4 groups). The large extraframework cages contain water molecules and are linked to each other via eight-membered rings (similar to 5 Angstrom in diameter). Two distinct Na cations are located: one in the off-center position of the eight-membered ring and another in the internal corner of cage. Crystal data: Na-4[(TiO)(4)(SiO4)(3)] . H2O, M-r = 719.8, rhombohedral, space group R3m (No. 160), a = 7.8124(6) Angstrom, alpha = 88.794(9)degrees, V = 476.50(6) Angstrom(3), Z = 1, R-p = 0.09, R-wp = 0.12. (C) 1997 Academic Press.