Spin states and hyperfine interactions of iron incorporated in MgSiO3 post-perovskite

Abstract

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 ~120 to 135GPa. 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 (LS) 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)O3 Ppv, we find the combination of HS Fe3+ at the A site and LS 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 Mössbauer 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.