A tensor LEED determination of the structure and compositional profile of a Cu{100}-c(2x2)-Pt surface alloy

The geometric structure and compositional profile of aCu{100}-c(2 x 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 x 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 (Theta(pt) = 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 Angstrom (Deltadz(12) = +1.9 +/- 1.1%), 1.91 +/- 0.03 Angstrom (Deltadz(23) = +5.8 +/- 1.7%) and 1.89 +/- 0.03 Angstrom (Deltadz(34) = + 4.7 +/- 1.7%). The rippling in the first mixed CuPt monolayer is small and of amplitude 0.03 +/- 0.04 Angstrom with Pt rippled outwards towards the solid-vacuum interface. (C) 2002 Elsevier Science B.V. All rights reserved.