Spin-state crossover and hyperfine interactions of ferric iron in MgSiO$_3$ perovskite

TL;DR

This study uses density functional theory to investigate the spin states of ferric iron in MgSiO$_3$ perovskite, revealing a pressure-induced crossover at the B-site that impacts mineral properties relevant to Earth's lower mantle.

Contribution

It provides the first detailed theoretical analysis of the spin-state crossover of ferric iron in MgSiO$_3$ perovskite under high pressure conditions.

Findings

B-site ferric iron transitions from high-spin to low-spin state between 40-70 GPa.

Volume reduction associated with the crossover affects seismic properties.

Results align with recent experimental measurements.

Abstract

Using density functional theory plus Hubbard $U$ calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O$_3$ perovskite, a major mineral phase in the Earth's lower mantle, has high-spin ferric iron ($S=5/2$) at both the dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site iron undergoes a spin-state crossover to the low-spin state ($S=1/2$), while the A-site iron remains in the high-spin state. Our calculation shows that the B-site spin-state crossover in the pressure range of 40-70 GPa is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent X-ray diffraction and M\"ossbauer spectroscopy measurements. The volume reduction leads to a significant softening in the bulk modulus, which suggests a possible source of seismic velocity anomalies in the lower mantle.