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

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. Plain Language Summary Under Earth's deep lower mantle and inner core pressures and temperatures, iron silicide (FeSi) takes a CsCl‐type crystalline structure (B2‐FeSi). It has been proposed that B2‐FeSi is a potential component in the ultralow‐velocity zones (ULVZs) at the base of the lower mantle and in the inner core. Here, we apply first‐principles molecular dynamics simulations to determine the density and elastic wave velocities of B2‐FeSi under deep Earth conditions and compare its properties to seismic observations. At core‐mantle boundary conditions, a mixture of B2‐FeSi with the ambient mantle exhibits elastic behaviors consistent with the observations of ULVZs, suggesting its possible presence. However, at inner core conditions, B2‐FeSi and H‐bearing B2‐FeSi have substantially higher bulk sound velocity than that of the seismic model, invalidating its presence in the inner core as a major component. Key Points B2‐FeSi shows markedly lower velocities than those of the PREM at the base of lower mantle but higher velocities at inner core conditions The presence of H in B2‐FeSi reduces both the P‐ and S‐wave velocities of B2‐FeSi, but has little effect on the bulk sound velocity Thermoelastic properties of B2‐FeSi are more consistent with its presence in the ultra‐low veloc