New high-pressure phases of Fe7N3 and Fe7C3 stable at Earth's core conditions: evidences for carbon–nitrogen isomorphism in Fe-compounds

Abstract

We carried out ab initio calculations on the crystal structure prediction and determination of P–T diagrams within the quasi-harmonic approximation for Fe₇N₃ and Fe₇C₃. Two new isostructural phases Fe₇N₃-C2/m and Fe₇C₃-C2/m which are dynamically and thermodynamically stable under the Earth's core conditions were predicted. The Fe₇C₃-C2/m phase stabilizes preferentially to the known h-Fe₇C₃ at 253–344 GPa in the temperature range of 0–5000 K, and the Fe₇N₃-C2/m stabilizes preferentially relative to the β-Fe₇N₃ – at ∼305 GPa over the entire temperature range. This indicate that carbon and nitrogen can mutually coexist and replace each other in the Earth's and other planetary cores similarly to low pressure phases of the same compounds.