Interionic potentials from ab initio molecular dynamics: The alkaline earth oxides CaO, SrO, and BaO

Abstract

Transferable potential models of interatomic interactions in CaO, SrO, and BaO are obtained by fitting the forces and stress tensor given by the aspherical ion model (AIM) [J. Chem. Phys. 108, 10209 (1998)] to those obtained from the ab initio molecular dynamics code CASTEP on condensed phase ion configurations generated at different values of pressure, temperature, and coordination environment. The AIM allows for dipole and quadrupole polarization of the ions as well as for ion-shape deformation due to short-range interactions. The potentials produce thermal expansivities, elastic properties, and phonon dispersion curves of these materials in excellent agreement with experiment. The BaO potential is also found to reproduce the complicated pressure-driven phase transitions observed in experiments on this material, even though these structures were not represented in the fitting procedure. The fitting process shows the increasing significance of cation polarization and shape deformation for the interionic interactions on descending the alkaline earth group, and this becomes an essential feature to account for the stability of the observed high-pressure phases of BaO.