A comparative study of the ¿-phases of iron nitrides and iron carbides from first-principles

First-principles calculations were performed for the c-phases and other iron carbides/nitrides with hexagonal close-packed Fe sublattices. Although these nitrides/carbides have similar crystal structures, they exhibit different chemical and physical properties. Relative to alpha-Fe, graphite and N-2, all the epsilon-type nitrides are stable, while all the carbides are metastable. The lattice parameters of the epsilon-iron nitrides vary differently from those of the epsilon-carbides, as a function of the concentration of X (X=N, C). The structural relationships of epsilon-Fe2X with eta-Fe2X and zeta-Fe2X are discussed.