The sound velocity measurements of Fe3S

We measured the sound velocity of Fe3S at room temperature up to 85 GPa employing inelastic X-ray scattering to better constrain the constitution of the inner core. The density of Fe3S was also determined by X-ray diffraction under the same conditions. The relation of the P-wave velocity (v(P)) and density (rho) of Fe3S follows Birch's law, v(P)(m/s) = 1.14(5) x rho(kg/m(3)) - 2580(410). Based on Birch's law determined here for Fe3S and that for epsilon-Fe reported previously, we found that sulfur decreases both density and compressional velocity of hcp-Fe at the core pressure and 300 K.