GermanyThe adsorption of potassium on Al(lll) at 90 K and at 300 K has been investigated by low-energy electron diffraction (LEED). Although a (\/3 x y/3)R30° structure is formed at each temperature, a detailed LEED analysis has revealed that the adsorbate positions are quite different and unusual in each case. At 90 K the adatoms occupy on-top sites and at 300 K they occupy substitutional sites. An irreversible phase transformation from the former to the latter structure occurs on warming to 300 K. These results are discussed in the light of recent density-functional-theory calculations.

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 (v 3 x~3)R30' structure is formed, however, the adsorbate positions are different. For adsorption at 90 K the adatoms occupy on-top sites and at 300 K they occupy substitutional sites, both geometries have hitherto been regarded as very unusual. An irreversible phase transition from the former to the latter stable structure occurs on warming to 300 K. The obtained interatomic distances and surface relaxations are discussed in terms of an interplay of the electrostatic repulsion between adatoms and substrate screening as well as coordination number. An approximate scheme for including the effect of anisotropic atomic motions under the in8uence of thermal excitation was applied. It is found that the R factors for the on-top site are signi6cantly improved whereas no improvement is found for the substitutional site.

The atomic geometry of the Al(111)-(&3X &3)R30'-structure formed by adsorption of Na on Al (111) at room temperature has been determined by analysis of extensive low-energy electron-diffraction (LEED) measurements. It is shown conclusively that Na atoms occupy a sixfold-coordinate site formed by displacing every third Al atom in the first layer of the substrate. This confirms the results of a recent study by surface extended x-ray-absorption fine structure.

The surface structure of the Pd (100)- c (2×2)-K phase formed by adsorption of K at room temperature has been determined by quantitative analysis of low-energy electron-diffraction (LEED) intensity-energy measurements. K atoms occupy four-fold hollow sites on a slightly perturbed substrate. The vertical distance between the K layer and the first Pd layer is determined to be 2.13±0.06 Å, which corresponds to an effective hard-sphere radius of 1.83 Å for the adsorbed K atoms. The second Pd layer is rumpled with a splitting of 0.04 Å between two bilayers. An analysis of LEED intensities measured for the clean Pd(100) surface confirms previous reports of an expansion of the first interlayer spacing. Adsorption of K in the c (2×2) structure results in a reduction of this expansion from 5% to 1%.

The Al(111)-(2 X 2)-Na phase formed by adsorption of one-half monolayer Na at 300 K is shown to be a surface alloy. Quantitative agreement is achieved between the results of structure determinations by low energy electron diffraction, surface extended x-ray adsorption fine structure, and total energy calculations.