First-principles simulations were performed, within density functional theory, in order to investigate the effects of pressure upon the formation of defects (ionic vacancies) and ionic diffusion in the perovskite and post-perovskite phases of MgSiO3. The results showed that the predicted formation enthalpies of 3 Schottky (MgO, SiO2, MgSiO3) defects were similar for the 2 phases at 100 to 150GPa; with the MgO Schottky defect being the most favorable. The calculated activation enthalpies and activation volumes for diffusion were shown to differ substantially between them. In particular, the activation enthalpies for Mg and Si diffusion in post-perovskite phases were smaller than the corresponding values for perovskite phases, by factors of 2.2 and 3.4, respectively, at 120GPa. However, the O migration enthalpy of post-perovskite phases was only slightly larger than that of perovskite phases. The easy migration paths of the cations in post-perovskite phases were shown to be along the <100> direction, in which Si–O octahedra shared edges. Visualization of the simulation data revealed that the vacancy defects and migrating ions introduced substantial distortions into the atomic and electronic structures around them. It was suggested that diffusion was equally easy for all 3 species in post-perovskite phases and was likely to occur via extrinsic processes.

A Computational Study of Ionic Vacancies and Diffusion in MgSiO3 Perovskite and Post-Perovskite. B.B.Karki, G.Khanduja: Earth and Planetary Science Letters, 2007, 260[1-2], 201-11