Structural stability and magnetism of γ'-Fe4N and CoFe3N compounds

Total energy and electronic structure calculations for γ'-Fe4N and CoFe3N in the anti-perovskite crystal structure have been performed to understand the structural stability and magnetism using plane-wave pseudopotential method. The virtual crystal approximation (VCA) was used to study order/disorder to examine the effect of the substitution of Co for Fe on the structural stability in CoFe3N alloy. The results show that CoFe3N energetically prefers ordered configuration for Co atoms occupying the corner position. Moreover, the substitution of Co for Fe at corner sites does slightly change the lattice constant, whereas it increases bulk modulus and could hardly changes the ductility. To reveal the role of nitrogen in γ'-Fe4N and CoFe3N, the Mulliken charge and bond population are used as a measure of charge transfers and an analysis of the covalency of these systems, respectively. It turns out that the further distant atoms from the N atom have the larger magnetic moment for γ'-Fe4N and CoFe3N, due to the spin-down electrons transfer from the corner atoms to the face-centered atoms.