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 0.16 eV 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 surface 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.