SURFACE-ANALYSIS OF THE PDCU(110) SINGLE-CRYSTAL ALLOY AT DIFFERENT SEGREGATION RATES

The PdCu(110) plane, with Pd:Cu = 1:1 in the bulk, was prepared to have various surface compositions by applying cycles of argon ion sputtering (ion energy of less than or equal to 750 eV with ion current densities less than or equal to 1 mu A/cm(2)) and annealing temperatures (T-AN) Of 420 < T < 820 K. With this preparation method different compositions of the top surface layers (TL) were obtained: (i) exclusively Pd atoms, (ii) composition of various Cu/Pd ratios, including the bulk ratio; and (iii) exclusively Cu atoms. The surface region (SR) of similar to 4 layers depth analysed by AES, and the CO TDS used for TL-characterization, allowed the average ratio of Cu/Pd in the next three subsurface layers (3SSL) to be evaluated LEED enabled the determination of the surface structure after each step. The high temperature treatment of the surface followed by low temperature sputtering (T-SP) and T-AN below that of Cu segregation (T-SEG) similar to 550 K, produced a smooth Pd-rich surface. On the other hand, a large number of sputtering and annealing cycles without prior high T-AN produced a rough surface, enabling the Cu atoms lying beneath the TL to be accessible for surface reactions. A diagram describing LEED structures can be drawn from the results which present the large variety of surface structures for the clean PdCu(110) single crystal surface depending on the T-SP and T-AN in addition to this diagram the SR Cu/Pd ratio for Various T-SP and T-AN is given for slow and fast annealing modes at long and short pretreatment periods. Three phase diagrams comparing experimental surface properties for each of the SR, TL and 3SSL with bulk composition are presented.