High-Pressure Phase Stability and Thermoelastic Properties of Iron Carbonitrides and Nitrogen in the Deep Earth

Iron-dominant metallic phases are likely the primary hosts for nitrogen in the reduced deep Earth, hence the storage of nitrogen in the lower mantle and the core is governed by the behavior of the Fe-N-C system at high temperatures and pressures. In this study, phase transitions and thermoelastic properties of iron carbonitrides were investigated at high pressure-temperature conditions by diamond anvil cell experiments and first-principles calculations. Experimental data revealed no phase transition in epsilon-type Fe-4 (N0.6C0.4) or Fe-7 (N0.75C0.25)(3) up to 60 GPa at room temperature. At high temperature, Fe-7 (N0.75C0.25)(3) transforms into the Fe3C-type phase at similar to 27 GPa, and then into the Fe7C3-type phase at similar to 45 GPa, which is also corroborated by our theoretical calculations. We found that the phase stability of iron carbonitrides mainly depends on the N/C ratio, and the elastic properties of iron carbonitrides are dominantly affected by the Fe/(N+C) ratio. Iron carbonitrides with diverse structures may be the main host for nitrogen in the deep mantle. Some iron carbonitride inclusions in lower mantle diamonds could be the residue of the primordial mantle or originate from subducted nitrogen-bearing materials, rather than iron-enriched phases of the outer core. In addition, our experiments confirmed the existence of Fe7C3-type Fe7C3-Fe7N3 solid solutions above 40 GPa. Fe7C3-type Fe-7(C, N)(3) has comparable density and thermoelastic properties to its isostructural endmembers and may be a promising candidate constituent of the Earth's inner core.