Spin-polarized periodic density functional theory was used to examine C atom adsorption on, absorption in, and diffusion into Fe(110) and Fe(100). It was found that C atoms bonded strongly with Fe surfaces and preferred high coordination sites. The C atom was predicted to adsorb on the long-bridge site on Fe(110) and the fourfold hollow site on Fe(100). Due to the very short distance between the C atom and the subsurface Fe atom of Fe(100), the C atom binds more strongly with Fe(100) than with Fe(110). In the subsurface region, the C atom prefers the octahedral site, as in bulk Fe. It was found that the C atom was more stable in the subsurface octahedral site of Fe(110) than that of Fe(100), since the strain caused by the interstitial C atom was released by pushing one surface Fe atom towards vacuum by 0.5Å in Fe(110), while the distortion in Fe(100) propagates far into the lattice. Diffusion of C atoms into Fe(110) and Fe(100) sub-surfaces goes through transition states where the C atom was coordinated to four Fe atoms. The barriers to diffusion into Fe(110) and Fe(100) were 1.18eV and 1.47eV, respectively. The larger diffusion barrier into Fe(100) was mainly due to the stronger bonding between C and the Fe(100) surface. It was predicted that the rate-limiting step for C incorporation into bulk Fe was the initial diffusion to sub-surface sites, while the rate-limiting step for absorbed C segregation to the surface was bulk diffusion, with no expected difference between rates to segregate to different surfaces. Lastly, it was predicted that graphite formation would be more favorable on C-covered Fe(110) than C-covered Fe(100).

Carbon Atom Adsorption on and Diffusion into Fe(110) and Fe(100) from First Principles. D.E.Jiang, E.A.Carter: Physical Review B, 2005, 71[4], 045402 (6pp)