Phase relations of iron carbides Fe (sub 2) C, Fe (sub 3) , and Fe (sub 7) C (sub 3) at the Earth's core pressures and temperatures

Based on first-principle calculations in the framework of the density functional theory and structure prediction algorithms, we have determined iron carbide phases stable at the Earth's core pressures and temperatures. It is shown that Fe (sub 7) C (sub 3) is unstable and decomposes into the mixture Fe (sub 2) C + Fe (sub 3) C over the entire range of pressures and temperatures specific to the Earth's inner core. Subsequent decomposition of Fe (sub 3) C into the mixture Fe + Fe (sub 2) C is unfavorable. We also predict a new low-temperature modification Fe (sub 3) C-C2/m-II dynamically and thermodynamically stable over the pressure range 290-305 GPa.