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DFT calculations of various Fe4N and Fe4C superstructures

Thursday (29.09.2016)
11:45 - 12:00
Part of:

Fe4N is the most prominent iron nitride attracting attention as a product of various types of heat treatments of steel but also due to its ferromagnetic properties. In Fe4N, the Fe atoms show a face-centered cubic (fcc) arrangement with N occupying ¼ of the octahedral sites such that a cubic L12-like superstructure results [1]. Unequivocal experimental evidence for an isostructural Fe4C, however, lacks. Frequent consideration of an L12-like Fe4C in the literature might have been motivated by a report [2] of an Fe4C compound with energetically unlikely [3] tetrahedral site occupation, which has the same unit cell size as the L12 structure as well as by evidence for L12-like C ordering in C-rich zones of tetragonal Fe-C martensite [4]. This had led to many reports of DFT calculations of Fe4C with L12-like superstructure in the literature. Differences in the N–N and C–C interactions in short-ranged-ordered Fe-N/C austenite [5], however, suggest that differences should also exist in the ordering of Fe4N and Fe4C. Hence, we have now performed DFT calculations using the VASP software [6] on numerous fcc-based Fe4N and Fe4C superstructures, predicted as described in [7], to reveal the preferred state of ordering for either case.

These calculations reveal that for Fe4N the experimentally well-established L12-like superstructure is clearly the most stable one. However, for Fe4C this is not the case. Instead, several more stable C arrangements with C on the octahedral sites were encountered. Many of these structures experience a pronounced Bain distortion into the direction of a body-centred cubic arrangement of the Fe atoms. The structures of Fe4N and Fe4C and their energies are governed by interplay between strain-induced interactions and the ionic repulsion between the interstitial atoms. The stronger ionic interactions in the case of Fe4N appear to stabilise the L12-like superstructure, whereas for Fe4C with less electronegative C atoms, other superstructures come into play containing nearest-neighbour C–C pairs, which do not occur in the L12-like superstructure. Hence, if an fcc-based Fe4C would exist, it would be unlikely to show a L12-like superstructure.

[1] K.H. Jack, Proc. Roy. Soc. London A 195 (1948) 34.

[2] Z.G. Pinsker, S.V. Kaverin, Sov. Phys. Crystallogr. 1 (1956) 48.

[3] C.M. Deng, C.F. Huo, L.L. Bao, X.R. Shi, Y.W. Li, J. Wang, H. Jiao, Chem. Phys. Lett. 448 (2007) 83.

[4] G.B. Olson, M. Cohen, Metall. Trans. A 14A (1983) 1057-1065.

[5] V.G. Gavriljuk, H. Berns, High Nitrogen Steels, Springer, Berlin (1999).

[6] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15.

[7] G.L.W. Hart, R.W. Forcade, Phys. Rev. B 77 (2008) 224115.

Prof. Dr. Andreas Leineweber
Technische Universität Bergakademie Freiberg
Additional Authors:
  • Beshid A. Manavi
    Max-Planck-Institut für Eisenforschung, Düsseldorf
  • Dr. Sascha B. Maisel
    Max-Planck-Institut für Eisenforschung, Düsseldorf