SEGREGATION PROCESSES IN PDCU(110) AND THE EFFECTS OF SULFUR IMPURITYON SURFACE-COMPOSITION AND MICROSTRUCTURE FROM ANNEALING

Various surface compositions (in similar to 4 uppermost layers) are pr epared on a PdCu(110) single-crystal alloy. Low energy electron diffra ction, Auger electron spectroscopy and thermal desorption spectroscopy of CO are used for surface characterisation. Rejection of foreign ato ms (sulphur) from the matrix with increasing temperature or in CO at h igher pressure (similar to 10(-3) mbar) is accompanied by drastic chan ges of the Pd-Cu surface microstructure. Sulphur segregation is accomp anied by Cu co-segregation and reverse segregation, surface clustering , an increasing amount of defects on the surface and a decrease of sur face roughening temperature. The (1 x 2) structure in the Cu-rich and the Pd-rich surfaces changes by annealing into a Pd-rich (1 x 1) struc ture, and sulphur segregates from the bulk to the surface. Sulphur ato ms statistically bind on Pd atoms in the top layer at atomic concentra tion ratios S/Pd less than or equal to 0.2. For higher coverages sulph ur develops the commensurate c(2 x 2) overlayer and enriches in grain boundaries of substrate domains. For CO chemisorption there is a corre spondence between the electronegativity of the impurity atom and the i nhibition of surface adsorption and change of the binding energy. Besi des surface microstructure, long-range electronic effects also play an important role in the PdCu single-crystal alloy surface. (C) 1997 Els evier Science Ltd. All rights reserved.