Thermodynamics and mechanism of the B1-B2 Phase Transition in Group 1 Halides and Group 2 Oxides

We study the thermodynamics, mechanism, and kinetic aspects of the B1-B2 phase transition in alkali halides and alkaline-earth oxides, using both two-body potentials and first-principles periodic Hartree-Fock theory, including a posteriori correlation corrections and self-consistent density functional calculations. Both the Buerger and Watanabe-Tokonami-Morimoto mechanisms are shown to be operable, for the activation energies of the two mechanisms are very close. The activation energies predicted by the potential model are always smaller than those from first-principles electronic structure calculations. Transition paths based on both approaches show marked variations with pressure and can be used to rationalize the pressure hysteresis observed experimentally. We discuss the effects of changing cation and anion size and charge on the transition pressures and activation energies and volumes.