Ab initio study of MgSiO₃ and CaSiO₃ perovskites at lower-mantle pressures

We have used a newly developed ab initio constant-pressure molecular dynamics technique to investigate the zero-temperature behaviour of MgSiO₃ and CaSiO₃ perovskites up to pressures which exceed the highest values reached within the Earth's mantle. Despite the similarities between these solids, we demonstrate that their behaviours at 150 GPa differ: whereas CaSiO₃ prefers the cubic perovskite phase, MgSiO₃ remains in the orthorhombically distorted perovskite (Pbnm) phase. Our theoretical strategy, besides in principle allowing finite-temperature simulations of solids under isotropic pressure, permits efficient and accurate determination of structural behaviour under arbitrary external pressures and/or stresses. This enabled us to offer reliable predictions for the elastic constants and shear modulus of MgSiO₃ perovskite, and to provide an estimate of the bulk modulus and its pressure derivative for CaSiO₃. Our calculations show that a Ca-enriched lower mantle would have a similar compressibility to a Ca-poor one.