Lattice dynamics and structural distortions of CaSiO₃ and MgSiO₃ perovskites

The phonon spectra, distortion mechanisms, and thermoelastic properties of CaSiO₃and MgSiO₃ in the cubic perovskite structure are investigated as a function of pressure using a lattice dynamic approach. The bonding forces are derived from a parameter‐free rigid‐ion electron‐gas formulation. At low pressures, CaSiO₃ is found to be dynamically stable in the cubic perovskite structure; however, the phonon spectrum exhibits soft modes at the Brillouin zone boundary which ultimately result in a dynamic instability of the lattice near 80 GPa. The computed phonon spectrum of cubic MgSiO₃ perovskite exhibits complex frequencies along parts of the zone boundary at all densities investigated. These vibrational instabilities include coupled octahedral rotations which produce the observed distorted structure of MgSiO₃ perovskite. The measured bulk modulus of MgSiO₃ perovskite compares well with our calculated value for the cubic structure.