Structure and stability of the crystal Fe₂C and low index surfaces

Spin-polarized density functional theory (DFT) calculations have been performed on the structure and stability of Fe₂C. It is found that orthorhombic Fe₂C is more stable than hexagonal Fe₂C by 0.16eV on the basis of the computed cohesive energies. The structures and stability of the orthorhombicFe₂C low index surfaces have also been investigated at the same level and the low index surfaces have the decreased stability order of (011) > (110) > (100) > (101) > (001). Comparison of the most stable Fe₃C, Fe₄C and Fe₂C surfaces shows that there is no linear correlation of surface energy and carbon content. And comparison of their most stable surfaces with the body-centered cubic Fe shows that these carbide surfaces have lower surface energies than the most stable (110) surface of body-centered cubic Fe, indicating that the surface thermodynamics favor carburization at Fe surfaces.