Ab initio lattice dynamics of MgSiO3 perovskite at high pressure

Phonon dispersions of MgSiO3 perovskite are calculated as a function of pressure up to 150 GPa using density-functional perturbation theory. Predicted zone-center frequencies and their pressure shifts are in close correspondence with existing Raman and infrared data, even though identification of the measured modes may be ambiguous. It is shown that the frequencies increase monotonically with pressure and no soft modes exist over the pressure regime studied. Low-frequency modes appear to be primarily associated with the SiO6 octahedral libration and large Mg displacement whereas high-frequency modes are dominated by octahedral deformation. The calculated frequencies are then used to determine the thermal contribution to the Helmholtz free energy within the quasiharmonic approximation and derive the equation of state, heat capacity, and entropy.