O adsorption and incipient oxidation of the Mg(0001) surface

First-principles density-functional calculations are used to study the early oxidation stages of the Mg(0001) surface for oxygen coverages 1/16<~Θ<~3 monolayers. It is found that at very low coverages O is incorporated below the topmost Mg layer in tetrahedral sites. At higher oxygen load the binding in on-surface sites is increased but at one monolayer coverage the on-surface binding is still about 60 meV weaker than for subsurface sites. The subsurface octahedral sites are found to be unfavorable compared to subsurface tetrahedral sites and to on-surface sites. At higher coverages oxygen adsorbs both under the surface and up. Our calculations predict island formation and clustering of incorporated and adsorbed oxygen in agreement with previous calculations. The calculated configurations are compared with results from angle-scanned x-ray photoelectron diffraction experiments to determine the geometrical structure of the oxidized Mg(0001) surface.