We have developed a simple ab initio model for the calculation of the thermoelastic properties of ionic compounds. The model is based on the Gordon-Kim-type electron-gas theory with spherically symmetric relaxation of ionic charge densities. The relaxation is controlled by a spherically averaged potential due to the total crystal charge density. The potential is self-consistent with the charge distribution, and contains Coulomb, exchange, and correlation contributions. We find that this potential yields anions that are slightly smaller than those stabilized by a point-ion Coulomb potential only. In the case of MgO, this results in a zero-pressure density that differs from experiment by less than 1%, a significant improvement over models that include only point-ion stabilization potentials. Further, the calculated equations of state and B1-B2 phase-transition pressures of NaCl, KCl, MgO, CaO, and SrO are in equally good agreement with data. The calculated equation of state and structure of the more covalent and less symmetric ${\mathrm{SiO}}_2$ stishovite is only slightly less accurate.

• Received 1 March 1991

DOI:https://doi.org/10.1103/PhysRevB.44.2495