The iron–nitrogen system: the crystal structures of ε-phase iron nitrides

At compositions near FesN and Fern the structures proposed for e iron nitrides by previous workers from incomplete X-ray data are confirmed. At intermediate nitrogen concentrations and at concentrations less than that of FesN, additional superlattice reflexions, unaccounted for by these previously proposed structures, are observed. Over the complete e range, the true unit of structure (space group D]-C312) has dimensions a' --~ (2V3)a, c' = c; where a and c are the dimensions of the approximately close-packed hexagonal unit of the iron-atom arrangement. The nitrogen atoms tend towards maximum separation and achieve considerable ordering at all concentrations within the phase limits. The proposed structures are correlated with the observed unit cell dimensional changes.