Ab initio structural predictions for ultrathin aluminum oxide films on metallic substrates

On several metallic substrates, first-principles density-functional calcula tions of Al2O3 5-7 Angstrom films predict a structure, stabilized by interf acial electrostatics, which has no bulk counterpart. In two and three O-lay er films, Al ions prefer distorted tetrahedral sites, over the normal octah edral sites. The film/substrate interface is found to consist of strongly c hemisorbed oxygen and is the determining factor in geometry; these oxygens are only weakly perturbed by the presence of Al2O3 overlayers. In an experi mentally relevant film (two O layers, as on NiAl), Al ions are nearly copla nar with surface O ions; thus observing "oxygen termination'' does not indi cate a polar surface. As a test case, we find Pt overlayers bind similarly as to alpha-Al2O3, suggesting these films may indeed be useful models for t hicker materials. [S0163-1829(99)50524-9].