Ordered mixed surface structures formed on Cu(0 0 1) by coadsorption of dissimilar metals: (2√2 × √2)R45° by Mg and Li, and (√5 × √5)R26.7° by Mg and K(Cs)

The coadsorption of Mg and alkali-metals (Li, K or Cs) on Cu(0 0 1) at room temperature has been studied by low-energy electron diffraction (LEED). A (2√2 × √2)R45° structure is formed by Mg and Li at coverages both about 0.25 ML, and which has been determined by a tensor LEED analysis. It is an ordered mixed phase, in which Mg atoms are located in fourfold substitutional sites and Li atoms occupy fourfold surface hollow sites, forming alternative one-dimension-like atomic rows along the [0 1 0] direction. On the other hand, a (√5 × √5)R26.7° structure is formed by the coadsorption of Mg with either K or Cs at coverage about 0.2 ML each. These structures have also been determined: Mg atoms are in fourfold substitutional sites, while K (Cs) atoms are on surface hollow sites, forming square arrangements of Mg and K (Cs). It is found that the top layer Cu atoms in both ordered structures are displaced laterally, and that Mg atoms are located at deeper positions in fourfold substitutional sites than in a c(2 × 2) substitutional structure formed by individual Mg adsorption. The reason for the formation of the ordered mixed surface structures is discussed.