Structures and stability of ABO3 orthorhombic perovskites at the Earth's mantle conditions from first-principles theory

We report first-principles calculations on the structures and stabilities of the ABO3 (A = Mg and Ca; B = Si and Ge) orthorhombic perovskites (OPvs) under high pressures. Calculations have also been performed for CdGeO3 and CdTiO3 OPvs. The calculations showed that MgSiO3, MgGeO3, CaGeO3, CdGeO3 and CdTiO3 OPvs transform to the orthorhombic post-perovskites (OPPvs) at about 101, 47, 55, 78 and 64 GPa, respectively, while CaSiO3 OPv is stable under high pressures. The theoretical results are in good agreement with the available experiments. The lattice distortions with pressure have been studied by analyzing the lattice deviations from the corresponding cubic perovskite (CPv). The lattice distortions increase with pressure for MgSiO3, MgGeO3, CaGeO3 and CdTiO3 OPvs, while CaSiO3 OPv has very small lattice distortions under high-pressures and the lattice distortions of CdGeO3 OPv even decrease with increasing pressure. The OPPvs have large distortions for the a-axis (about −10%).