A SATLEED study of the geometric structure of Cu{100}-Pd monolayer surface alloys

The structure of a Cu{100}-p(2 x 2) surface alloy formed by deposition of 1 ML of Pd on Cu{100} 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 x 2) CuPd alloy with p(2 x 2)p2gg symmetry introduced into the outermost layer via clock rotation of the Cued monolayer with the corners of the p(2 x 2) unit cell centred over second layer Pd atoms (RP = 0.21). Lateral shifts of the top layer Cu and Pd atoms are determined to be 0.25 +/- 0.12 Angstrom. 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 Angstrom (+6.6 1.1%) and 1.90 +/- 0.03 Angstrom (+5.3 +/- 1.7%) and a rippling of Pd and Cu atoms in the outermost layer of 0.06 +/- 0.03 Angstrom 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 {100}-c(2 x 2) -Pd underlayer alloy leads to a structure which retains a simpler c(2 x 2) periodicity, suggesting that the growth of the p(2 x 2)-glide line phase requires a c(2 x 2) CuPd outermost template. (C) 2002 Elsevier Science B.V. All rights reserved.