LATTICE-ENERGY CONFORMATIONAL-ANALYSIS OF ORTHOSILICATE CA3OSIO4 (C3S) - REGULARIZING CA-SUBLATTICE VERSUS DEFORMING SIO44- ANIONS

Orthosilicate Ca3OSiO4, and other related compounds, preserves the fee or hcp structure of metallic Ca, where some Ca are substituted by SiO 4 and the oxygens fill the octahedral holes. The SiO44- anions deform the cubooctahedral cage Ca-12 around them, and this deformation propag ates along the crystal. Both the preferred orientations of SiO4, and t he induced deformations of the Ca sublattice, which were previously ob served by XRD, are now predicted by simple lattice-energy conformation al analysis, where the match is improved when the potential model incl udes weak Ca-Ca covalent bonds between neighboring cations. The polyty pism of Ca3OSiO4 is explained by the presence of different arrangement s of the SiO4 orientations in the crystal. The transition between poly types with the temperature being of the order-disorder type. (C) 1996 Academic Press, Inc.