Vibrational and quasiharmonic thermal properties of CaO under pressure

We have investigated by first principles the phonon dispersions of CaO across its pressure induced phase transformation from the B1 (NaCl structure) to the B2 (CsCl structure) phase. Beyond their stability fields we have uncovered phonon instabilities that should be associated with this phase transition. We have also used these dispersions in conjunction with the quasiharmonic approximation (QHA) to calculate several thermodynamic properties in both phases. Comparisons between the predicted properties with experimental measurements and with similar properties of MgO indicate that the QHA is quite unsatisfactory for CaO. Despite this, the calculated transition pressure at room temperature falls within the range of measurements. This transition is shown to have a negative Clapeyron slope.