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

a r t i c l e i n f o Keywords: Fe-bearing post-perovskite (MgSiO 3) ferrous and ferric iron (Fe 2 + and Fe 3 +) spin state crossover quadrupole splittings equation of state lower mantle D″ layer LDA + U Using density functional theory + Hubbard U (DFT + U) calculations, we investigate the spin states and nuclear hyperfine interactions of iron incorporated in magnesium silicate (MgSiO 3) post-perovskite (Ppv), a major mineral phase in the Earth's D″ layer, where the pressure ranges from ~120 to 135 GPa. In this pressure range, ferrous iron (Fe 2 +) 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 (Fe 3 +), 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 Fe 3 + at the A site and LS Fe 3 + at the B site the most favorable. Neither A-site nor B-site Fe 3 + undergoes a spin-state crossover in the D″ pressure range. The computed iron quadru-pole splittings are consistent with those observed in Mössbauer spectra. The effects of Fe 2 + and Fe 3 + on the equation of state of Ppv are found nearly identical, expanding the unit cell volume while barely affecting the bulk modulus. Iron, the most abundant transition-metal element in the Earth, is widely present in major mantle minerals, including olivine, pyroxene, garnet, ferropericlase, magnesium silicate (MgSiO 3) perovskite (Pv), and the recently discovered MgSiO 3 post-perovskite (Ppv) with the CaIrO 3-type (Cmcm) structure (Murakami et al., 2004; Oganov and Ono, 2004; Tsuchiya et al., 2004). Owing to its incomplete 3d electron shell and comparable crystal field splitting energy and Hund's exchange energy, the total electron spin moment S of iron in minerals can vary with pressure and temperature. This phenomenon, known as spin-state crossover, has a great influence on physical and chemical properties of the host minerals. A well studied example is ferroperi-clase [(Mg,Fe)O], the second most abundant mineral in the lower mantle. In this mineral, ferrous iron (Fe 2 +) undergoes a spin-state crossover from high-spin (HS), S = 2, to low-spin (LS), S = 0, at pressures near the mid mantle (45–55 GPa), which strongly affects the compressibility (Badro et al. also reduces the radiative thermal …