An unusual bimetallic p4g phase induced by Pd on Cu(001)

The surface composition and structure of Pd on Cu(001) have been studied by a combination of low-energy ion scattering (LEIS) and low-energy electron diffraction (LEED). Palladium is incorporated into the surface at room temperature (less than or equal to 50 degrees C). This leads to the formation of two surface alloys: an ordered c(2 x 2) structure at 0.5 ML Pd, and a p4g structure at 1.0 ML Pd. The surface composition of the top two layers was determined by 1 keV Li+ ion scattering, using selective scattering geometries with calibration measurements on reference samples of Cu(001) and Pd(001). For the p4g structure, the results show a surface composition of 53at%Cu-47at% Pd in the first layer, while the second layer is 58at%Cu-42at%Pd. Several structural models that could be responsible for the p4g structure are proposed and investigated. The randomly mixed top Cu-Pd layer was found to be reconstructed such that the Pd (Cu) atoms are rotated by a lateral displacement of 0.25+/-0.10 Angstrom. The magnitude of this displacement was determined by measuring the azimuthal distributions of the Pd-Pd double scattering peak with 2 keV K+ ions. It is proposed, based on a hard-sphere model, that it is the moderate size mismatch between Cu and Pd atoms in the second c(2 x 2) mixed layer which drives the clockwise-anticlockwise rotation.