Modeling thermodynamics of Fe-N phases; characterisation of e-Fe2N1-z

In order to arrive at modeling the thermodynamics of Fe-N phases, including long-range (LRO) and short-range ordering (SRO) of the N atoms, it is important to understand the role of N interstitially dissolved in an Fe-host lattice. The crystal structure of -Fe2N1-z consists of an h.c.p. iron sublattice and a hexagonal nitrogen sublattice formed by octahedral interstices of the Fe sublattice [1]. Two ground-state structures have been proposed for the ordered arrangement of the N atoms on their own sublattice [1], which were shown to be thermodynamically favourable [2]: configuration A for Fe2N1 (z=0) and configuration B for Fe2N2/3 (z=1/3). So far, only configuration B was identified unambiguously by diffraction techniques [1,3]. In the present work homogeneous -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 configurations of LRO of the N atoms [2,3] 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.