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

The coadsorption of Mg and alkali-metals (Li, K or Cs) on Cu(001) at room t emperature 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 bo th 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 sub stitutional sites and Li atoms occupy fourfold surface hollow sites, formin g alternative one-dimension-like atomic rows along the [0 1 0] direction. O n 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. Thes e structures have also been determined: Mg atoms are in fourfold substituti onal 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 b oth ordered structures are displaced laterally, and that Mg atoms are locat ed 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) 20 01 Elsevier Science B.V. All rights reserved.