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 (vP) and density (ρ) of Fe3S follows Birch’s law, vP(m/s) = 1.14(5) × ρ(kg/m3) − 2580(410). Based on Birch’s law determined here for Fe3S and that for ɛ-Fe reported previously, we found that sulfur decreases both density and compressional velocity of hcp-Fe at the core pressure and 300 K.

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