Spin-state crossover and hyperfine interactions of ferric iron in MgSiO3 perovskite

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 Mossbauer 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.