STABILITY AND ELASTICITY OF THE PEROVSKITE AND POST-PEROVSKITE PHASES AND IMPLICATIONS FOR THE LOWER MANTLE

We study from static first-principles calculations the perovskite-post-perovskite phase relation as a function of relevant chemistry for the Earth's lower mantle. We find that the transition takes place at 110 GPa for pure MgSiO₃. The addition of Al₂O₃ slightly increases this transition pressure, while the addition of Fe²⁺ considerably reduces this transition pressure, the FeSiO₃ end-member term being stable in the post-perovskite modification with respect to perovskite at all pressures. We also determine the equations of state, the densities, the elasticity and the seismic wave velocities. We find that at the transition, Vp has a small jump while Vs has a large jump, consistent with the seismic observations from the D". The addition of both Fe²⁺ and Al₂O₃ decrease the seismic wave velocities of MgSiO₃ aggregates. For MgSiO₃ we report a detailed analysis of the lattice dynamical properties and the Raman intensities of both perovskite and post-perovskite structures and we investigate the stability of the post-perovkiste structure at pressures beyond the ones characteristic for the lower mantle.