A thorough investigation of the adsorption and diffusion of Mg, O and O2 on MgO(001) terraces was performed by using first-principles calculations. The single Mg adatom weakly bonded to surface O atoms, diffused and evaporated easily at room temperatures. Atomic O strongly bonded to surface O atoms; forming peroxide groups. The diffusion of the O adatom was strongly influenced by the spin polarization, since energy barriers were significantly different for the singlet and triplet states. The crossing of the 2 Born-Oppenheimer surfaces corresponding to the distinct spin states was also analyzed. Although the O2 molecule does not stick to the perfect surface, it chemisorbs on surface non-stoichiometric point defects such as O vacancies or Mg adatoms, forming in the latter case new chemical species on the surface. It was shown that the oxidation rate limiting factor in an O2 atmosphere was the concentration of point defects (O vacancies and Mg adatoms) in the growing surface. The simulated O core-level shifts for the various adsorption configurations enable a meaningful comparison with the measured values, suggesting the presence of peroxide ions on growing surfaces. Finally, the computed energy barriers were used to estimate the Mg and O surface lifetimes and diffusion lengths.

Adsorption and Diffusion of Mg, O and O2 on the MgO(001) Flat Surface. G.Geneste, J.Morillo, F.Finocchi: Journal of Chemical Physics, 2005, 122[17], 174707