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

Carbon has been suggested as one of the light elements existing in the Earth's core. Under core conditions, iron carbide Fe (sub 7) C (sub 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 (sub 7) C (sub 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 (sub 7) C (sub 3) up to 80 GPa and 800 K. Our diffraction data indicate that Fe (sub 7) C (sub 3) adopts an orthorhombic structure under experimentally investigated conditions. The pressure-volume-temperature data for Fe (sub 7) C (sub 3) were fitted by the high-temperature Birch-Murnaghan equation of state, yielding ambient-pressure unit-cell volume V (sub 0) = 745.2(2) Aa (super 3) , bulk modulus K (sub 0) = 167(4) GPa, its first pressure derivative K (sub 0) ' = 5.0(2), dK/dT = -0.02(1) GPa/K, and thermal expansion relation alpha (sub T) = 4.7(9) X 10 (super -5) + 3(5) X 10 (super -8) X (T - 300) K (super -1) . We also observed anisotropic elastic responses to changes in pressure and temperature along the different crystallographic directions. Fe (sub 7) C (sub 3) has strong anisotropic compressibilities with the linear moduli Ma > Mc > Mb from zero pressure to core pressures at 300 K, rendering the b axis the most compressible upon compression. The thermal expansion of c (super 3) is approximately four times larger than that of a (super 3) and b (super 3) at 600 and 700 K, implying that the high temperature may significantly influence the elastic anisotropy of Fe (sub 7) C (sub 3) . Therefore, the effect of high temperature needs to be considered when using Fe (sub 7) C (sub 3) to explain the anisotropy of the Earth's inner core.