Study of the iron nitride FeN into the megabar regime

In this work, the nitrogen-rich portion of the Fe-N binary phase diagram is investigated up to 128 GPa. The samples, largely in excess of nitrogen, were laser-heated in diamond anvil cells to temperatures up to 2000 K at regular pressure intervals to help in crossing possible activation barriers towards the more stable phase. Three Fe-N compounds: ZnS-type FeN, Fe2N and NiAs-type FeN, are characterized by powder X-ray diffraction and their observed stability domain reported. Below 12.5 GPa, orthorhombic Fe2N is found to be the energetically-favored compound while NiAs-FeN becomes stable above 17.7 GPa. Energy-dispersive X-ray spectroscopy measurements and a Rietveld refinement confirmed the stoichiometry and structure of the recovered NiAs-FeN sample. A precise determination of its bulk modulus (K0 = 200(5) GPa) as well as its pressure derivative (K0′ = 5.3(2)) is obtained and, based on its unit cell axial ratio evolution, the NiAs-FeN compound appears to decrease in ionicity concomitantly with pressure. Within the pressure-temperature conditions reached here, the predicted iron pernitride FeN2 is not observed.