Spin crossover of iron in aluminous MgSiO₃ perovskite and post-perovskite

Using density functional theory+Hubbard U (DFT+U) calculations, we investigate how aluminum affects the spin crossover of iron in MgSiO₃ perovskite (Pv) and post-perovskite (Ppv), the major mineral phases in the Earth's lower mantle. We find that the presence of aluminum does not change the response of iron spin state to pressure: only ferric iron (Fe³⁺) in the octahedral (B)-site undergoes a crossover from high-spin (HS) to low-spin (LS) state, while Fe³⁺ in the dodecahedral (A)-site remains in the HS state, same as in Al-free cases. However, aluminum does significantly affect the placement of Fe³⁺ in these mineral phases. The most stable atomic configuration has all Al³⁺ in the B-site and all Fe³⁺ in the A-site (thus in the HS state). Metastable configurations with LS Fe³⁺ in the B-site can happen only at high pressures and high temperatures. Therefore, experimental observations of LS Fe³⁺ at high pressures in Al-bearing Pv require diffusion of iron from the A-site to the B-site and should be sensitive to the annealing temperature and schedule. In the Earth's lower mantle, the elastic anomalies accompanying the B-site HS-LS crossover exhibited in Al-free Pv are likely to be considerably reduced, according to the B-site Fe³⁺ population.