LEED STRUCTURAL-ANALYSIS OF AL(111)-K-(ROOT-3-X-ROOT-3)-R30-DEGREES -IDENTIFICATION OF STABLE AND METASTABLE ADSORPTION SITES

A detailed low energy electron diffraction analysis has been performed to investigate the adsorption of potassium on Al(111) at 90 K and at 300 K. At each temperature a (square-root 3 x square-root 3)R30-degree s structure is formed, however, the adsorbate positions are different. For adsorption at 90 K the adatoms occupy on-top sites and at 300 K t hey occupy substitutional sites, both geometries have hitherto been re garded as very unusual. An irreversible phase transition from the form er to the latter stable structure occurs on warming to 300 K. The obta ined interatomic distances and surface relaxations are discussed in te rms of an interplay of the electrostatic repulsion between adatoms and substrate screening as well as coordination number. An approximate sc heme for including the effect of anisotropic atomic motions under the influence of thermal excitation was applied. It is found that the R fa ctors for the on-top site are significantly improved whereas no improv ement is found for the substitutional site.