Prediction of cell variations with pressure of ionic layered crystal Application to the matlockite family

Compounds belonging to the matlockite family are ionic layered crystals. Some previous experimental studies on these compounds demonstrated the correlation between the difference in bond strength translating the layered properties and the anisotropic coordination of the highly polarizable halogen anion. In the present paper, we present a model to estimate the anisotropic compressibilities of matlockites which could be generalized to other ionic layered compounds. The compressibilities of each individual polyhedron which built the sheet of an ionic layered crystal are determined from a simple relation based on a semi-empirical relation obtained by Hazen et al., involving the electrical charge of the ions and the interatomic distances. The electrical charge of the anions are taken to be direction-dependent and are modulated by the anion polarizability value. Finally, to validate the model, the calculated compressibilities are compared with previously published data, but also with new high pressure experimental data obtained by X-ray absorption spectroscopy up to 20 GPa on SrFCl and SrFBr at the K edge of the strontium and by X-ray diffraction up to 30 GPa on SrFBr and PbFBr. Present estimations give results which differ from the experimental values by less than 15%. Received 26 October 1998