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

epsilon '-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 epsilon-Fe allotrope. The observation of an anomaly in the composition-dependent evolution of the lattice parameters of low-N content (at approximate to Fe5N) epsilon '-iron nitrides evident from previously published lattice parameter data motivated us to perform first-principles calculations on the nonmagnetic and ferromagnetic structures of epsilon-Fe and epsilon '-Fe6N and epsilon '-Fe3N superstructures, the latter presenting epsilon '-iron nitrides. These calculations reveal a low-volume nonmagnetic ground state of epsilon-Fe and high-volume ferromagnetic ground states for the considered epsilon '-Fe6N and epsilon '-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 epsilon '-iron nitrides and also the previously observed characteristics in the composition-, temperature-, and state-of-order dependences of the axial ratio of epsilon '-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.