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Carbon has been suggested as one of the light elements existing in the Earth’s core. Under core conditions, iron carbide Fe 7 C 3  is likely the first phase to solidify from a Fe-C melt and has thus been considered a potential component of the inner core. The crystal structure of Fe 7 C 3 , however, is still under debate, and its thermoelastic properties are not well constrained at high pressures. In this study, we performed synchrotron-based single-crystal X-ray diffraction experiment using an externally heated diamond-anvil cell to determine the crystal structure and thermoelastic properties of Fe 7 C 3  up to 80 GPa and 800 K. Our diffraction data indicate that Fe 7 C 3  adopts an orthorhombic structure under experimentally investigated conditions. The pressure-volume-temperature data for Fe 7 C 3  were fitted by the high-temperature Birch-Murnaghan equation of state, yielding ambient-pressure unit-cell volume  V  0  = 745.2(2) Å 3 , bulk modulus  K  0  = 167(4) GPa, its first pressure derivativeK   0    ′$\begin{array}{} \displaystyle K_{0}^{\prime} \end{array}$= 5.0(2), d K /d T  = -0.02(1) GPa/K, and thermal expansion relation α  T   = 4.7(9) × 10 –5  + 3(5) × 10 –8  × ( T  – 300) K –1 . We also observed anisotropic elastic responses to changes in pressure and temperature along the different crystallographic directions. Fe 7 C 3  has strong anisotropic compressibilities with the linear moduli  M a   &gt;  M c   &gt;  M b   from zero pressure to core pressures at 300 K, rendering the  b  axis the most compressible upon compression. The thermal expansion of  c  3  is approximately four times larger than that of  a  3  and  b  3  at 600 and 700 K, implying that the high temperature may significantly influence the elastic anisotropy of Fe 7 C 3 . Therefore, the effect of high temperature needs to be considered when using Fe 7 C 3  to explain the anisotropy of the Earth’s inner core.

The high-pressure anisotropic thermoelastic properties of a potential inner core carbon-bearing phase, Fe 7 C 3 , by single-crystal X-ray diffraction