First-principles analysis on the role of rare-earth doping in affecting nitrogen adsorption and diffusion at Fe surface towards clarified catalytic diffusion mechanism in nitriding

The adsorption and diffusion of nitrogen atom on the Fe surface and in Fe subsurface with and without substitution of La and Ce in the surface layer were studied based on first-principles calculations. Nitrogen atoms are predicted to be adsorbed at hollow and long bridge sites on Fe(100) and Fe(110), respectively. The nitrogen atoms on Fe surfaces respectively with doped La and Ce in subsurface layer show similar trends. However, the nitrogen atom preferentially occupies the hollow site on both Fe(100) and Fe(110) with La or Ce doped in the surface layer. La doping in the surface layer lowers the adsorption energies of both Fe(100) and Fe(110), indicating that La doping endows stronger adsorption ability. The effect of the La doping on the adsorption energies was explained based on the configurations after relaxation. In the subsurface region, the nitrogen atom prefers to occupy the octahedral site in Fe slabs with and without La and Ce doping, respectively. Furthermore, La and Ce doping in surface layer stabilizes the system of Fe slab with nitrogen atom as a foreign interstitial. The penetration process of the nitrogen atom into Fe(100) surface layers with and without La or Ce doping as a rate-determining step were calculated and compared. This study provides a fundamental insight into the catalytic diffusion mechanism for nitriding iron surface involving rare-earth surface doping. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.