Effect of site degeneracies on the spin crossovers in (Mg, Fe)SiO₃ perovskite

We investigate by first principles several aspects of the spin crossover of ferrous iron (Fe²⁺) between the high-spin (HS), intermediate-spin (IS), and low-spin (LS) states in MgSiO₃ perovskite under pressure. First, we investigate the effect of site degeneracy intrinsic to LS and IS states of iron. We show that the entropy owing to this effect (site entropy) significantly increases the fraction of LS irons and reduces the crossover onset pressure, especially at high temperatures. Even including the effect of site entropy, the fraction of IS irons in the lower mantle or in the laboratory is predicted to be negligible. We also show that the effect of the crossover on equation-of-state parameters is probably undetectable at any temperature. Although we address only ferrous iron in only one atomic configuration and results are specific to this situation, our results suggest that the effect of the crossover on the bulk modulus derived from direct sound speed measurements could perhaps be more noticeable below room temperature. However, at mantle conditions the bulk sound speed should hardly be affected by the crossover.