Sapphire (0001) Surface, Clean and withd-Metal Overlayers

We present local density-functional results for structural and electronic properties of ${\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$, clean and with Pt and Ag adsorption. Significant surface relaxations penetrate to the third oxygen layer, 5.2 Å below the surface. The dominant mechanism of metal adhesion is polarization and is relatively weak ( $\simeq 0.4$ and 0.6 eV/atom for Ag and Pt, respectively); however, isolated metal atoms bind up to $5\times$ as strongly with an ionic bond induced by the surface Madelung potential.