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

摘要

Orthosilicate Ca3OSiO4, and other related compounds, preserves the fee or hcp structure of metallic Ca, where some Ca are substituted by SiO4 and the oxygens fill the octahedral holes. The SiO44- anions deform the cubooctahedral cage Ca-12 around them, and this deformation propagates along the crystal. Both the preferred orientations of SiO4, and the induced deformations of the Ca sublattice, which were previously observed by XRD, are now predicted by simple lattice-energy conformational analysis, where the match is improved when the potential model includes weak Ca-Ca covalent bonds between neighboring cations. The polytypism of Ca3OSiO4 is explained by the presence of different arrangements of the SiO4 orientations in the crystal. The transition between polytypes with the temperature being of the order-disorder type. (C) 1996 Academic Press, Inc. [References: 13]