To investigate the physical property of Fe7C3, we carried out in situ X-ray diffraction experiments using a Kawai-type multi-anvil apparatus and a diamond anvil cell up to 71.5 GPa and 1973 K. The carbide was found to be stable under these experimental conditions. However, we found anomalous behavior in its isothermal compression and thermal expansivity. These anomalies could be due to the magnetic phase transition in Fe7C3 from a ferromagnetic (fm) to a paramagnetic (pm) phase. The Curie temperature of 523 K at 1 bar (Tsuzuki et al. 1984) decreases with pressure, and the pressure-induced magnetic transition is estimated to occur at ~18 GPa and 300 K. The pressure-volume-temperature (P-V-T) data set for the pm-Fe7C3 was fitted by the Mie-Grüneisen-Debye (MGD) equation of state (EOS) and the following parameters were obtained: unit-cell volume V0 = 184.2 ± 0.3 Å3, bulk modulus K0 = 253 ± 7 GPa, the pressure derivative of bulk modulus K0 = 3.6 ± 0.2, Grüneisen parameter γ0 = 2.57 ± 0.05, Debye temperature 𝛉0 = 920 ± 140 K, and q = 2.2 ± 0.5, respectively, at zero pressure. The calculated density for Fe7C3 provides a good explanation for the density of the Earth’s inner core obtained from seismological observations.

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