First-principles study on the mechanical, electronic and magnetic properties of Fe3C

The structural, mechanical, electronic and magnetic properties of Fe3C with hexagonal structure and orthorhombic structure were investigated by using first-principles technique. The internal positions of atoms within the unit cell were optimized and the ground state properties such as lattice parameter, elastic constants, bulk modulus and the final enthalpy of the Fe3C in hexagonal and orthorhombic structure were calculated. The results show that the calculated equilibrium structural parameters of Fe3C are in agreement with the experimental results well, and the formation energy of Ortho-Fe3C is lower than that of Hexa-Fe3C, but the mechanical stability of Ortho-Fe3C is lower than that of Hexa-Fe3C. The density of states (DOSs) reveals that both Ortho-Fe3C and Hexa-Fe3C are strong metallic carbide. The calculated electronic structure indicates that the bonds of Fe3C are of the mixtures of metallic, covalent, and ionic properties. The average magnetic moments (Ms) of the Ortho-Fe3C and Hexa-Fe3C are 1.396 and 1.497 μB/atom, respectively. The C atoms are oppositely polarized in the two crystals as indicated by the spin-density maps. For Ortho-Fe3C, the Fe Ms values of 1.915 and 1.990 μB for Fe′ and Fe′′ sites were obtained, respectively. This difference on Ms between two different Fe sites in Ortho-Fe3C indicates that the Fe moments are sensitive to the local short-range order in the crystal.