Structural, electronic, and magnetic properties of iron carbide Fe7C3 phases from first-principles theory

The iron carbide Fe7C3 exhibits two types of basic crystal structures, an orthorhombic (o-) form and a hexagonal (h-) one. First-principles calculations have been performed for the basic Fe7C3 forms and for the related theta-Fe3C cementite phase. Accurate total-energy calculations show that the stability of Fe7C3 is comparable to that of theta-Fe3C. The o-Fe7C3 phase is more stable than the hexagonal one, in contrast to recent atomistic simulations. Furthermore, the calculations also show a rather low energy for a carbon vacancy in the o structure, which implies possible C deficiency in the lattice. Both Fe7C3 phases are ferromagnetic metals. Electronic band-structure calculations show that all Fe atoms exhibit high-spin states with the majority of their 3d states being almost fully occupied. From an analysis of the structural and energetic properties, the formation of the o phase in steel treatment processes and of h form in carburization of ferrite is discussed.