IDENTIFICATION OF STABLE AND METASTABLE ADSORPTION SITES OF K ADSORBED ON AL(111)

The adsorption of potassium on Al(111) at 90 K and at 300 K has been investigated by low-energy electron diffraction (LEED). Although a (square-root 3 x square-root 3)R30-degrees 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.