Using low-energy electron diffraction structural analysis and first-principles calculations based on the local-density approximation we have investigated the $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ K and $(2\mathrm{}\times 2)$ K overlayers on Pt{111}. The measured and calculated adsorption geometries agree quantitatively. In both phases the K adatoms occupy threefold symmetric hcp hollow sites, the preference for the hcp site over the fcc site being a consequence of the polarization of the surface Pt $d$ electrons. We have not found any indication of K incorporation into the Pt{111} surface, as has been recently suggested.

• Received 23 September 1997

DOI:https://doi.org/10.1103/PhysRevB.57.9246