Substitution mechanism of Al ions in MgSiO₃ perovskite under high pressure conditions from first-principles calculations

Large-scale first-principles density functional theory (DFT) calculations have been carried out to investigate how Al³⁺ can be incorporated into MgSiO₃ perovskite under high pressure and to study the resultant change in the compressional mechanism of MgSiO₃ perovskite. We examined two types of MgSiO₃ models with 6.25 mol% Al₂O₃: charge-coupled substitution and oxygen-vacancy mechanisms. Five pressure points from 0 to 100 GPa have been considered. At each pressure point, we have calculated five models of the oxygen vacancy and five models of the charge-coupled mechanisms. We also change the internal positions of the substituted Al in the calculated cells, which have 80 atoms. Our free energy calculations show Al³⁺ replaces the nearest-neighbor cation pairs (Mg²⁺ and Si⁴⁺) at all pressures investigated. The calculated bulk modulus of the most energetically favorable model is 3.4% lower than that of the Al-free MgSiO₃ perovskite. These results may have important implications for discriminating between thermal and compositional effects of 1-D Earth models and the possible influence of aluminum perovskite.