Fe-N System at High Pressures and Its Relevance to the Earth's Core Composition

Based on ab initio calculations within the density functional theory and crystal structure prediction algorithms, the structure and stability of iron-nitrogen compounds in the pressure range of 100-400 GPa and temperatures up to 4000 K were determined. Three new iron nitrides Fe4N3-Imm2, Fe2N-Pnma, and Fe3N-C2/m were predicted. Fe4N3 was shown to be stable at pressures up to 266 GPa and then decompose into Fe2N + 2FeN. Predicted Fe2N-Pnma becomes stable with respect to the decomposition reaction 9Fe(2)N = Fe4N3 + 2Fe(7)N(3) at pressures above 221 GPa. Fe3N-C2/m stabilizes with respect to decomposition into 2Fe + Fe7N3 at pressures above 265 GPa. Also, it was shown that beta-Fe7N3 synthesized in diamond anvil cell experiments has an orthorhombic Pbca structure, and at pressures above similar to 320 GPa decomposes into 2Fe(2)N + Fe3N. All predicted Fe-rich iron nitrides, except Fe4N3-Imm2, have structural analogs among iron carbides. Considering the temperature effect, we observed that FeN-P2(1)3, Fe2N-Pnma, and Fe3N-C2/m can be stable at the Earth's inner core pressures and temperatures up to 4000 K, whereas Fe4N3-Imm2 and beta-Fe7N3 are thermodynamically unstable in the entire studied temperature range. Although Fe7N3-Pbca is thermodynamically unstable at inner core pressures, it shows the closest coincidence of the S- and P-wave velocities with seismic observations among the studied Fe-nitrides. Overall, Fe-nitrides cannot be the major compounds in the inner core of the Earth and can only substitute other elements such as carbon in Fe-carbides in minor amounts.

Automated summary

Ab initio calculations and crystal structure prediction algorithms were used to determine the structure and stability of iron-nitrogen compounds under different pressure and temperature conditions. Three new iron nitrides were predicted. Some iron nitrides can be stable under Earth's inner core pressures and temperatures, but they are not the major compounds in the inner core.