A tensor LEED determination of the structure and compositional profile of a Cu{1 0 0}-c(2×2)-Pt surface alloy

The geometric structure and compositional profile of a Cu{1 0 0}-c(2×2)-Pt surface alloy formed by thermal activation of a monolayer Pt film has been determined by tensor low energy electron diffraction (LEED). A wide range of models have been tested. The favoured model consists of an ordered c(2×2) CuPt underlayer below a Cu terminated surface. Models involving a mixed ordered CuPt layer outermost may be definitively ruled out. The average T-matrix approximation (ATA) has been applied allowing variable Pt concentrations to be introduced into both the outermost layer and deeper into the selvedge (layers 3 and 4) in the form of a random substitutionally disordered CuxPt1−x alloy. The favoured concentration profile corresponds to an almost pure outermost Cu monolayer (ΘPt=10±10 at.%) with Pt concentrations of 20±20 and 30±30 at.% in layers 3 and 4 respectively. Introduction of Pt into the surface layers induces a significant expansion of the selvedge yielding modification of the outermost three interlayer spacings to 1.84±0.02 Å (Δdz12=+1.9±1.1%), 1.91±0.03 Å (Δdz23=+5.8±1.7%) and 1.89±0.03 Å (Δdz34=+4.7±1.7%). The rippling in the first mixed CuPt monolayer is small and of amplitude 0.03±0.04 Å with Pt rippled outwards towards the solid–vacuum interface.