Energies and interactions in binary (Pbnm) orthosilicates: A Born parametrization

Abstract

A structure-energy model valid for ionic orthosilfcates with orthorhombic symmetry is presented. The structure simulation is achieved with a distance-least-square modelling (DLS; Baerlocher et al., 1977) of cell edges and spacial coordinates in the asymmetric unit. The input parameters are based on multiple regressions of the mean effective radii of cations occupying the M1 and M2 positions. The cohesive energy is then evaluated as the sum of coulombic + repulsive + dispersive terms. The parametrization of the repulsive energy is achieved with the Huggins-Mayer form ( cf. Tosi, 1964). The enthalpy at 298.15° K, 1 bar is then calculated with the Born-Haber-Fayans thermochemical cycle. Gibbs free energy values at different T conditions are then evaluated for various states of intracrystalline disorder and the stable configuration is selected, based on the principle of minimization of the Gibbs free energy of the system at equilibrium. The model is applied to 7 binary joins on which experimental data are available, to ascertain its consistency. The depicted solid solution properties, the calculated Gibbs free energy and the site occupancy predictions are in satisfactory agreement with experimental observations.