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

Abstract

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. In the ~-Fe~N structure the iron atoms retain the same relative positions as in e-Fe2N, but because of a rearrangement of the nitrogen atoms, which in ~ are packed more closely in one direction, the iron-atom lattice is anisotropically distorted. A mechanism for the e-F%N --> ~-F%N transition is suggested. A discussion of the structural features of the iron-nitrogen system indicates that the interstitial- atom ordering shown by all phases except one, and particularly by the a" and e phases, and the sharp change from one ordered arrangement (8) to a different but equally ordered interstitial-atom arrangement (~) cannot be ascribed to ordinary close-packing or size factors. Introduction