Thermodynamics and long-range order of interstitials in an h.c.p. lattice: Nitrogen in ε-Fe2N1 − z

The thermodynamics of nitrogen in the ε-Fe2N1 − z phase were evaluated. To this end absorption isotherms for nitrogen in ε-Fe2N1 − z, depicting the dependence of the nitrogen content on the applied chemical potential of nitrogen, were determined in the temperature range 673–823 K by equilibrating iron foils in gaseous ammonia/hydrogen mixtures. The absorption isotherms could be described very well by a Gorsky-Bragg-Williams approximation for long-range order (LRO) of nitrogen atoms on the hexagonal nitrogen sublattice, formed by the octahedral interstices of the h.c.p. sublattice of iron atoms. The actual occurrence of LRO was evidenced by diffraction analysis. Mössbauer spectra were interpreted as composed of spectra of iron atoms surrounded by 1–3 nitrogen atoms. The relative amounts of the iron environments as determined from the Mössbauer spectra agreed very well with the corresponding probabilities predicted by the LRO model.