Vibrational and thermodynamic properties of MgSiO₃ postperovskite

Phonon dispersions and vibrational density of states of MgSiO₃ postperovskite, the new high-pressure phase of MgSiO₃ perovskite, are calculated as a function of pressure up to 180 GPa using density-functional perturbation theory. The calculated frequencies are then used to determine the thermal contribution to the Helmholtz free energy within the quasi-harmonic approximation. The equation of state and several thermodynamic properties of interest are derived and compared with those of perovskite. The overall thermodynamic properties of postperovskite are almost the same as those of perovskite under its stability conditions.