Pressure-induced magnetic transition and sound velocities of Fe3C: Implications for carbon in the Earth's inner core

We have carried out nuclear resonant scattering measurements on 57Fe‐enriched Fe3C between 1 bar and 50 GPa at 300 K. Synchrotron Mössbauer spectra reveal a pressure‐induced magnetic transition in Fe3C between 4.3 and 6.5 GPa. On the basis of our nuclear resonant inelastic X‐ray scattering spectra and existing equation‐of‐state data, we have derived the compressional wave velocity VP and shear wave velocity VS for the high‐pressure nonmagnetic phase, which can be expressed as functions of density (ρ): VP(km/s) = −3.99 + 1.29ρ(g/cm3) and VS(km/s) = 1.45 + 0.24ρ(g/cm3). The addition of carbon to iron‐nickel alloy brings density, VP and VS closer to seismic observations, supporting carbon as a principal light element in the Earth's inner core.