A SATLEED study of the geometric structure of Cu [formula omitted]-Pd monolayer surface alloys

Abstract

The structure of a Cu {1 0 0} -p(2×2) surface alloy formed by deposition of 1 ML of Pd on Cu {1 0 0} at room temperature has been studied by symmetrised automated tensor low energy electron diffraction. The favoured model from the wide range tested consists of a double layer ordered c(2×2) CuPd alloy with p(2×2)-p2gg symmetry introduced into the outermost layer via clock rotation of the CuPd monolayer with the corners of the p(2×2) unit cell centred over second layer Pd atoms ( R p=0.21). Lateral shifts of the top layer Cu and Pd atoms are determined to be 0.25±0.12 Å. Substitution of 0.5 ML of Pd in both layers 1 and 2 leads to a significant expansion of the outermost two interlayer spacing to 1.93±0.02 Å (+6.6±1.1%) and 1.90±0.03 Å (+5.3±1.7%) and a rippling of Pd and Cu atoms in the outermost layer of 0.06±0.03 Å with top layer Pd atoms rippled outwards. This model is in agreement with previous ion scattering studies of a Cu:Pd stoichiometry of 1:1 in the outermost two layers. A second mode of film growth consisting of adsorption of 0.5 ML of Pd on a copper capped Cu {1 0 0} -c(2×2)-Pd underlayer alloy leads to a structure which retains a simpler c(2×2) periodicity, suggesting that the growth of the p(2×2)-glide line phase requires a c(2×2) CuPd outermost template.