Ab initio lattice dynamics of MgSiO3 perovskite at high pressure

Phonon dispersions of MgSiO3 perovskite are calculated as a function of pre ssure up to 150 GPa using density-functional perturbation theory. Predicted zone-center frequencies and their pressure shifts are in close corresponde nce with existing Raman and infrared data, even though identification of th e measured modes may be ambiguous. It is shown that the frequencies increas e monotonically with pressure and no soft modes exist over the pressure reg ime 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 e nergy within the quasiharmonic approximation and derive the equation of sta te, heat capacity, and entropy.