Hexagonal Formula Presented-Iron Nitrides Occurring at Low Nitrogen Contents: Interplay between Nitrogen Content and Magnetism

Formula Presented′-Iron nitrides are interstitial metallic nitrides that attract interest due to their hardness and (ferro)magnetic properties. Their crystal structure is derived from the ordered insertion of N into a hexagonally close-packed arrangement of Fe atoms and thus from the high-pressure Formula Presented-Fe allotrope. The observation of an anomaly in the composition-dependent evolution of the lattice parameters of low-N content (at ≈ Fe₅N) Formula Presented′-iron nitrides evident from previously published lattice parameter data motivated us to perform first-principles calculations on the nonmagnetic and ferromagnetic structures of Formula Presented-Fe and Formula Presented′-Fe₆N and Formula Presented′-Fe₃N superstructures, the latter presenting Formula Presented′-iron nitrides. These calculations reveal a low-volume nonmagnetic ground state of Formula Presented-Fe and high-volume ferromagnetic ground states for the considered Formula Presented′-Fe₆N and Formula Presented′-Fe₃N superstructures. Moreover, the calculations indicate the characteristic effects of magnetic ordering on the volume and axial ratio of the unit cell, which explain the mentioned experimentally observed anomaly in the lattice parameters of low-N Formula Presented′-iron nitrides and also the previously observed characteristics in the composition-, temperature-, and state-of-order dependences of the axial ratio of Formula Presented′-iron nitrides. The type of evaluation of experimental and predicted lattice parameters highlights how far the careful evaluation of lattice parameters and their anisotropic evolution allows detecting subtle chemical-bonding interactions (in particular magnetic ones) in crystalline solids.