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 (57)Fe-enriched Fe(3)C between 1 bar and 50 GPa at 300 K. Synchrotron Mossbauer 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 V(P) and shear wave velocity VS for the high-pressure nonmagnetic phase, which can be expressed as functions of density (rho): V(P)(km/s) = -3.99 + 1.29 rho(g/cm(3)) and V(S)(km/s) = 1.45 + 0.24 rho(g/cm(3)). The addition of carbon to iron-nickel alloy brings density, V(P) and V(S) closer to seismic observations, supporting carbon as a principal light element in the Earth's inner core.