We determined the compressional velocity of hcp-Fe in a wide pressure and temperature range using high-resolution inelastic X-ray scattering (IXS) combined with in situ X-ray powder diffraction (XRD) on samples in resistively heated diamond anvil cells: our measurements extend up to 174 GPa at room temperature, to 88 GPa at 700 K, and to 62.5 GPa at 1000 K. Our data obtained at room temperature and high temperature are well described by a linear relation to density, extending the range of verification of Birch’s law beyond previous work and suggesting only a small temperature dependence up to 1000 K. When we compare the present results with the preliminary reference Earth model (PREM), we can conclude that there is either a strong temperature effect on Birch’s law at temperatures above 1000 K, or the composition of the core is rather different from that commonly expected, i.e., containing heavy elements.

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