Composition Dependence of Pressure-Induced Spin Transitions in the (Mg,Fe)SiO3 Perovskite and (Mg,Fe)O Rocksalt System

Recent experimental results suggest that Fe undergoes a high-spin to low-spin transition in both the rocksalt and perovskite phases at lower mantle pressures. These spin transitions may have a profound impact on the properties of lower mantle phases. In this work the critical spin-transition pressures for Fe in perovskite (Mg,Fe)SiO$_{3}$ and rocksalt (Mg,Fe)O are calculated using \textit{ab initio} methods. We focus in particular on the alloy nature of the material, studying the spin-transition pressure for varying concentrations of Fe. The results show that as the concentration of Fe increases, the transition pressure decreases in the perovskite. This is directly opposite the trend observed for spin transition pressures found in rocksalt (Mg,Fe)O. The difference in trends in spin-transition pressure is explained by the difference in physics between the two structures.