Temperature-dependent segregation reversal and (1 x 3) missing-row structure of Pt90Co10(110)

The surface structure and composition of the clean Pt90Co10(110) surface ar e investigated by low-energy electron diffraction (LEED) and low-energy ion scattering (LEIS). Through LEED I-V analysis, we End a (1 x 3) missing-row reconstruction on the equilibrated Pt90Co10(110) surface - comparable with the pure Pt(110) (1 x 3) surface in which all atomic positions in the topm ost layer and in the: (Ill)oriented microfacets are Pt-enriched. Due to the fact that the unreconstructed Pt90Co10(110) surface is known to exhibit an almost pure Co top layer, the Pr segregation reported in this study is und oubtedly connected to the existence of the missing-row reconstruction. The proposed structural influence on the composition is confirmed by LEIS exper iments performed on the hot Pt90Co10(110) surface, in which simultaneously temperature-induced changes of the surface composition and qualitative chan ges in the surface structure are monitored. The measured low-energy ion spe ctra not only reproduce the calculated first-layer composition of the LEED analysis but also show a less pronounced Pt segregation at temperatures aro und 750 degrees C. and eventually a reversed Pt segregation above 750 degre es C, i.e. Co enrichment of the Pt90Co10(110) surface with respect to the b ulk concentration. We find a clear correlation between the thermal deconstr uction and the surface composition. The striking segregation reversal durin g temperature variation is attributed to the high excess value of the mixin g enthalpy, which implies a structure-dominated segregation behavior. (C) 2 000 Elsevier Science B.V. All rights reserved.