New high-pressure phases of FeN and FeC stable at Earth's core conditions: evidences for carbon-nitrogen isomorphism in Fe-compounds.

We carried out calculations on the crystal structure prediction and determination of - diagrams within the quasi-harmonic approximation for FeN and FeC. Two new isostructural phases FeN-2/ and FeC-2/ which are dynamically and thermodynamically stable under the Earth's core conditions were predicted. The FeC-2/ phase stabilizes preferentially to the known h-FeC at 253-344 GPa in the temperature range of 0-5000 K, and the FeN-2/ stabilizes preferentially relative to the β-FeN - 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.