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

Using density functional theoryþHubbard U (DFTþU) 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 (Fe3þ) in the octahedral (B)-site undergoes a crossover from high-spin (HS) to low-spin (LS) state, while Fe3þ in the dodecahedral (A)-site remains in the HS state, same as in Al-free cases. However, aluminum does significantly affect the placement of Fe3þ in these mineral phases. The most stable atomic configuration has all Al3þ in the B-site and all Fe3þ in the A-site (thus in the HS state). Metastable configurations with LS Fe3þ in the B-site can happen only at high pressures and high temperatures. Therefore, experimental observations of LS Fe3þ 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 Fe3þ population. & 2012 Elsevier B.V. All rights reserved.