Thermoelastic Properties of B2-Type FeSi Under Deep Earth Conditions: Implications for the Compositions of the Ultralow-Velocity Zones and the Inner Core

Abstract

The CsCl-type (B2) phase of FeSi (B2-FeSi) has been proposed as a candidate phase in the ultralow-velocity zones (ULVZs) at the base of the lower mantle and in the Earth's inner core. However, the elastic properties of B2-FeSi under relevant conditions remain unclear. Here we determine the density, elastic constants, and velocities of B2-FeSi at high pressures (90–390 GPa) and temperatures (3,000–6,000 K) relevant to the Earth's lower most mantle and the inner core, using first-principles molecular dynamics simulations. At the base of the lower mantle, B2-FeSi shows significantly lower velocities and a higher density than those of the ambient mantle. Mechanical mixing models suggest the presence of ∼27–39 vol% B2-FeSi in the silicate mantle is consistent with the reduced velocities and the elevated density of ULVZs observed seismically. On the other hand, the hcp-Fe and B2-FeSi mixture exhibits higher bulk sound velocity compared to the PREM under inner core conditions. Adding superionic H in the interstitial sites of B2-FeSi lowers its density but has little effect on the bulk sound velocity of B2-FeSi, precluding H-bearing B2-FeSi as a major component in the Earth's inner core.