Adsorption of Al, O, Hf, Y, Pt, and S atoms on α-Al₂O ₃(0001)

We use first-principles density functional theory to investigate adsorption of Al, O, Hf, Y, Pt, and S-atoms on the α-Al₂O ₃(0001) surface. We identify stable adsorption sites and predict binding energies and structures. We find that Al, Hf, and Y preferentially adsorb on threefold-hollow sites, transfer electrons to the surface, and form ionic bonds to the three oxygen atoms. In contrast, the most stable adsorption site for Pt and S is ontop an oxygen atom, and we do not observe significant charge transfer. We find a binding order of S < Pt < O < Al ≪Y < Hf, which reflects both the ease with which the early transition metals Hf and Y ionize, as well as the (nearly) closed-shell repulsions influencing the adsorption of O, Pt, and S. We use these results to rationalize some observations regarding the stability of thermal barrier coatings.