Electronic and structural properties of cementite-type M3X (M=Fe, Co, Ni; X=C or B) by first principles calculations

Density functional theory within the generalized gradient approximation (GGA) is used to investigate the electronic structure and formation energies of cementite-like carbides and borides of Groups VII–VIII transition metals (TMs) (M3X; where M=Fe, Co, Ni; X=C or B). All phases are metallic-like; Fe3C, Co3C, Fe3B and Co3B exhibit ferromagnetic states whereas Ni3C and Ni3B are paramagnetic. The formation energies of M3X calculated with respect to bulk TMs, graphite and α-boron (B12) are negative for borides, however, they are positive for carbides, i.e. these M3C phases are metastable or unstable at ambient conditions. The stability differences of these compounds are discussed in terms of their electronic structure and chemical bonding.