Synthesis and structure analysis of the potassium calcium silicate CAS-1.: Application of a texture approach to structure solution using data collected in transmission mode

The crystal structure of the potassium calcium silicate material, CAS-1 (vertical bar(Ca4K4(H2O)(8))vertical bar[Si16O38]), has been determined from synchrotron powder diffraction data collected on a textured polycrystalline sample. It was not possible to solve the structure directly from normal high-resolution powder diffraction data, so the newly developed texture approach to structure solution using data collected in transmission mode was applied. Data analysis was performed using the computer program Expol, which has been developed to handle the large amount of data generated in transmission mode. The structure could then be solved by direct methods, and Rietveld refinement (using data collected on an untextured sample) converged with R-F = 0.062 and R-wp = 0.175 (R-exp = 0.097). This structure is the first novel structure to be solved using the texture method in this configuration. As suggested by Si-29 MAS NMR results (1 Q(4) and 3 Q(3) signals, all of equal intensity), CAS-1 (C2, a = 24.158 angstrom, b = 7.016 angstrom, c = 6.482 angstrom, beta = 95.19 degrees) proved to be a layer silicate. The structure can be described as a series of silicate double layers connected via chains of edge-sharing CaO5(H2O) octahedra. There is a two-dimensional eight-ring channel system within the silicate layer, and K+ ions, which balance the charge, are located in the eight-rings of the two 4.8(2) nets that form the double layer. One water molecule is located in the channel system and another bridges between K+ and Ca2+ ions. The structure is related to that of the natural mineral rhodesite. (c) 2005 Academie des sciences. Published by Elsevier SAS. All rights reserved.