Modeling thermodynamics of Fe-N phases: characterisation of epsilon-Fesub2Nsub1-x

In the present work homogeneous epsilon-nitride powders prepared at 723 K, having nitrogen contents ranging from 26.1 at. % N (z=0.29) to 31.1 at.% N (z=0.10), were investigated with X-ray diffraction (XRD) and Mössbauer spectroscopy. A thermodynamic model accounting for the two possible configurations of LRO of the N atoms was fitted to the N-absorption isotherm at 723 K and resulted in the occupancies of the sites of the nitrogen sublattice. A miscibility gap between configurations A and B was predicted. The XRD results did not allow for an unambiguous identification of configuration A, because the discriminating superstructure reflections (with respect to h.c.p. Fe) associated with configuration A were not observed. On the other hand, the probabilities of Fe atoms surrounded by 1 up to 3 N atoms, as derived from the Mössbauer spectra, are consistent with the distribution of nitrogen over the available sites and the occurrence of a two phase region A+B as predicted by the model. The discrepancy between XRD and Mössbauer results is discussed in terms of a reason for extinguished superstructure reflections in the X-ray diffractogram of configuration A.