Ordered mixed surface structures formed on Cu(001) by coadsorption of dissimilar metals:: (2√2x√2)R45° by Mg and Li, and (√5x√5)R26.7° by Mg and K(Cs)

The coadsorption of Mg and alkali-metals (Li, K or Cs) on Cu(001) at room temperature has been studied by low-energy electron diffraction (LEED). A (2 root2 x root2)R45 degrees 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 (root5 x rootS)R26.7 degrees 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 x 2) substitutional structure formed by individual Mg adsorption. The reason far the formation of the ordered mired surface structures is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.