Crystal structure and thermal behaviour of boropollucite CsBSi2O6

The crystal structure Of Cs0.82B1.09Si1.98O6 boropollucite at room temperature was determined by direct methods and refined in the 1a (3) over bard space group using an anisotropic approximation of atomic thermal displacements (a = 13.009 (1) Angstrom, Z = 16, R-W = 0.027, R-F = 0.037 for 141 independent observed (\F\ greater than or equal to 4sigma(F)) reflections). The occupancy factors have been refined for Cs and tetrahedral positions assuming the oxygen sites being fully occupied. The compound is isostructural to leucite-pollucite high-temperature modification. Thermal behaviour of CsBSi2O6 was investigated by DTA and TG, annealing at different temperatures with following wet chemical analysis and high-temperature X-ray powder diffraction methods. The CsBSi2O6 cubic phase loses mass before melting and decomposes to form a new crystalline phase with close to CsBSi3O8 stoichiometry in the temperature range of 1303-1353 K. Thermal expansion of two boropollucite samples, which differed in the number of Cs+ and/or B3+ ions of a nominal composition CsBSi2O6 was investigated in air as well as in vacuum. Temperature ranges of negative thermal expansion were found. (C) 2004 Elsevier SAS. All rights reserved.