Computational searches for iron carbide in the Earth's inner core

We have used density-functional-theory methods together with a structure searching algorithm to determine stable structures and stoichiometries of mixtures of iron and carbon at high pressures and zero temperature. The most favorable stoichiometries at Earth's inner-core pressures (similar to 350 GPa) are those with between about 20% and 35% carbon atoms. The most stable stoichiometries were found to be (Fe and C), Fe(3)C, Fe(7)C(3), and Fe(2)C. The latter has not to our knowledge been discussed previously in relation to the Earth's core. The stoichiometries Fe(4)C and Fe(5)C(2) were found to be close to stability at Earth's inner-core pressures. We find that Fe(7)C(3) is unstable to decomposition into Fe(3)C + 2Fe(2)C at pressures greater than similar to 330 GPa. At 150 GPa only Fe, C, Fe(3)C, and Fe(7)C(3) are stable. Formation of Fe/C compounds is energetically more favorable at 350 GPa than at 150 GPa. We also report a new phase for Fe(3)C with Cmcm symmetry to be more stable than the well-known cementite phase at 350 GPa. A number of pressure-induced phase transitions are identified in Fe(3)C, Fe(3)C(2), FeC, Fe(8)C, and FeC(2). The lowest enthalpy Fe/C phases were found to be metallic at the pressures studied.