Spin states and hyperfine interactions of iron incorporated in MgSiO 3 post-perovskite

Using density functional theory + Hubbard U (DFT + U) calculations, we investigate the spin states and nuclear hyperfine interactions of iron incorporated in magnesium silicate (MgSiO3) post-perovskite (Ppv), a major mineral phase in the Earth's D '' layer, where the pressure ranges from similar to 120 to 135 GPa. In this pressure range, ferrous iron (Fe2+) substituting for magnesium at the dodecahedral (A) site remains in the high-spin (HS) state; intermediate-spin (IS) and low-spin (IS) states are highly unfavorable. As to ferric iron (Fe3+), which substitutes magnesium at the A site and silicon at the octahedral (B) site to form (Mg,Fe)(Si,Fe)O-3 Ppv, we find the combination of HS Fe3+ at the A site and IS Fe3+ at the B site the most favorable. Neither A-site nor B-site Fe3+ undergoes a spin-state crossover in the D '' pressure range. The computed iron quadrupole splittings are consistent with those observed in Mossbauer spectra. The effects of Fe2+ and Fe3+ on the equation of state of Ppv are found nearly identical, expanding the unit cell volume while barely affecting the bulk modulus.