AN UNUSUAL BIMETALLIC P4G PHASE INDUCED BY PD ON CU(001)

The surface composition and structure of Pd on Cu(001) have been studi ed by a combination of low-energy ion scattering (LEIS) and low-energy electron diffraction (LEED). Palladium is incorporated into the surfa ce at room temperature (less than or equal to 50 degrees C). This lead s to the formation of two surface alloys: an ordered c(2 x 2) structur e at 0.5 ML Pd, and a p4g structure at 1.0 ML Pd. The surface composit ion 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 la yer, while the second layer is 58at%Cu-42at%Pd. Several structural mod els that could be responsible for the p4g structure are proposed and i nvestigated. The randomly mixed top Cu-Pd layer was found to be recons tructed such that the Pd (Cu) atoms are rotated by a lateral displacem ent of 0.25+/-0.10 Angstrom. The magnitude of this displacement was de termined by measuring the azimuthal distributions of the Pd-Pd double scattering peak with 2 keV K+ ions. It is proposed, based on a hard-sp here model, that it is the moderate size mismatch between Cu and Pd at oms in the second c(2 x 2) mixed layer which drives the clockwise-anti clockwise rotation.