Theoretical analysis of clock-reconstructed PdCu surface alloy

Ab initio density-functional techniques (DFT's) have been used to investigate the structural properties of PdCu surface alloys for which a clock reconstruction with p4g symmetry has ben reported. Our calculations not only allow to discriminate between the various models proposed on the basis of th experimental data, they also elucidate the physical mechanism leading to the clock-rotated p(2x2)-p4g surface structure. It is shown that free-standing square monolayers are unstable - not only relative to the most dense hexagonal lattice, but also relative to a square-triangle lattice characteristic of a p4g symmetry. This transformation is even barrier-less or limited by a low energetic barrier. Whether ths transformation will also occur in a surface layer depends on the coupling of the adlayer to the substrate - this explains why the reconstuction occurs only in the topmost layer of a bilayer PdCu alloy, but not n a single alloy layer. Our work demonstrate that DFT calculations provide a reliable tool for predicting surface reconstructions.