Surface alloys, surface rumpling and surface stress

The results of density functional theory calculations of the surface structure and surface stress of a series of 13 different surface alloy phases for which there are quantitative experimental structure determinations are presented, 12 involving alloying adsorbate atoms (Au, Pd, Bi, Sn, Mg, Pb) with larger atomic radii than those of the substrates (Cu, Ni, Pt, Rh) and one involving alloying elements with the opposite relative radii (W(100)c(2×2)–Cu). For the systems with larger adsorbate atoms the results confirm the experimental behaviour of reduced amplitudes of surface rumpling relative to those expected from simple hard-sphere arguments, but also show that the tensile surface stress is reduced by alloying, and indeed becomes strongly compressive for some systems. For the W(100)/Cu surface alloy, on the other hand, alloying increases the tensile surface stress. The results are discussed in the context of previous arguments based on the influence of valence electron charge density depletion at the surface.