New high-pressure phases of Fe(7)N(3) and Fe(7)C(3) stable at Earth's core conditions: evidences for carbon–nitrogen isomorphism in Fe-compounds

We carried out ab initio calculations on the crystal structure prediction and determination of P–T diagrams within the quasi-harmonic approximation for Fe(7)N(3) and Fe(7)C(3). Two new isostructural phases Fe(7)N(3)-C2/m and Fe(7)C(3)-C2/m which are dynamically and thermodynamically stable under the Earth's core conditions were predicted. The Fe(7)C(3)-C2/m phase stabilizes preferentially to the known h-Fe(7)C(3) at 253–344 GPa in the temperature range of 0–5000 K, and the Fe(7)N(3)-C2/m stabilizes preferentially relative to the β-Fe(7)N(3) – 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.