Prediction of Stable Iron Nitrides at Ambient and High Pressures with Progressive Formation of New Polynitrogen Species

Nitride materials are of considerable interest due to their fundamental importance and practical applications. However, synthesis of transition metal nitrides often requires extreme conditions, e.g., high temperature and/or high pressure, slowing down the experimental discovery. Using global structure search methods in combination with first principles calculations, we systematically explore the stoichiometric phase space of iron-nitrogen compounds on the nitrogen-rich side at ambient and high pressures up to 100 GPa. Diverse stoichiometries in the Fe-N system are found to emerge in the phase diagram at high pressures. Significantly, FeN4 is found to be stable already at ambient pressure. It undergoes a polymerization near 20 GPa which results in a high energy density. Accompanying the polymerization, FeN4 transforms from a direct band gap semiconductor to ferromagnetic metal. We also predict several phase transitions in FeN and FeN2 at high pressure, and the results explain the previous experimental observations by comparing the X-ray diffraction patterns. Stepwise formation of polynitrogen species is observed following the increment of nitrogen content in the stoichiometry, from isolated N atoms in FeN, to the N-2 unit in FeN2 and Fe3N8, to the N-6 unit in Fe3N8 and FeN3, and to the N-infinity chain in FeN4, FeN6, and FeN8. Ultra-incompressibility is found in marcasite-FeN2, FeN3, and FeN4 along particular crystalline directions, while high energy density, 1.37-2.02 kJ g(-1), is expected for FeN4, FeN6, and FeN8. Our results shed light on understanding the chemistry of transition metal polynitrides under pressure and encourage experimental synthesis of newly predicted iron nitrides in the near future.