title: Spin-state crossover and hyperfine interactions of ferric iron in MgSiO(3) perovskite.
abstract: Using density functional theory plus Hubbard U calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O(3) perovskite, the major mineral phase in Earth's lower mantle, has high-spin ferric iron (S=5/2) at both dodecahedral (A) and octahedral (B) sites. With increasing pressure, the B-site iron undergoes a spin-state crossover to the low-spin state (S=1/2) between 40 and 70 GPa, while the A-site iron remains in the high-spin state. This B-site spin-state crossover is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent x-ray diffraction and Mössbauer spectroscopy measurements. The anomalous volume reduction leads to a significant softening in bulk modulus during the crossover, suggesting a possible source of seismic-velocity anomalies in the lower mantle.
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, USA.
authors.forename: Han
authors.initials: H
authors.lastname: Hsu
authors.forename: Peter
authors.initials: P
authors.lastname: Blaha
authors.forename: Matteo
authors.initials: M
authors.lastname: Cococcioni
authors.forename: Renata M
authors.initials: RM
authors.lastname: Wentzcovitch