Equation of state and elasticity of B2-type FeSi: Implications for silicon in the inner core

Ab initio molecular dynamics simulations and high-pressure experiments were performed to investigate the equation of state and the elasticity of B2-type FeSi at core pressure-temperature conditions. The P-V-T relationship was determined by ab initio molecular dynamics calculations, corrected according to the high-pressure experimental data. All three independent elastic constants for B2-type FeSi, C11, C12, and C44, were calculated from the computation of changes in the free energy generated by small strains. The elastic constants were used to estimate the moduli and seismic wave velocities at the core pressure-temperature conditions. The estimated density of FeSi was lower than that of hcp-Fe, whereas the compressional and shear sound wave velocities of FeSi were higher than those of hcp-Fe. The inner core model for a simple mixture of Fe and FeSi was able to explain the density and seismic wave velocities of the Preliminary Reference Earth Model (PREM) determined from the seismic observations. However, the estimated temperature at the top of the inner core was too high, at ∼6500 K, for the inner core to become an iron-silicon solid alloy. This indicated that, if the inner core comprises two components, then silicon is excluded from the candidate light elements present in the inner core.