The neutral O vacancy energy formation for bulk, sub-surface sites at various depths from the surface and for various surface sites, was estimated for single crystals, unsupported ultra-thin films of MgO, CaO and BaO, and for MgO ultra-thin films supported on Ag(001). From the calculated energy barriers for diffusion through the surface, and from the surface to the bulk, it was found that diffusion was a hindered process; especially for MgO. Nevertheless, diffusion from the terrace to the step-edges was largely favored while diffusion through terrace sites was less likely, and surface-to-bulk migration had a very low probability. It was suggested that this explained recent scanning tunneling microscopic images for MgO thin films supported on Ag(001); which showed O vacancies preferentially populating the step-edge sites.

Bulk and Surface Oxygen Vacancy Formation and Diffusion in Single Crystals, Ultrathin Films, and Metal Grown Oxide Structures. J.Carrasco, N.Lopez, F.Illas, H.J.Freund: The Journal of Chemical Physics, 2006, 125[7], 074711 (6pp)