Temperature-dependent segregation reversal and (1×3) missing-row structure of Pt 90Co 10(110)

Abstract

The surface structure and composition of the clean Pt 90Co 10(110) surface are investigated by low-energy electron diffraction (LEED) and low-energy ion scattering (LEIS). Through LEED I– V analysis, we find a (1×3) missing-row reconstruction on the equilibrated Pt 90Co 10(110) surface — comparable with the pure Pt(110) (1×3) surface — in which all atomic positions in the topmost layer and in the (111) oriented microfacets are Pt-enriched. Due to the fact that the unreconstructed Pt 25Co 75(110) surface is known to exhibit an almost pure Co top layer, the Pt segregation reported in this study is undoubtedly connected to the existence of the missing-row reconstruction. The proposed structural influence on the composition is confirmed by LEIS experiments performed on the hot Pt 90Co 10(110) surface, in which simultaneously temperature-induced changes of the surface composition and qualitative changes in the surface structure are monitored. The measured low-energy ion spectra not only reproduce the calculated first-layer composition of the LEED analysis but also show a less pronounced Pt segregation at temperatures around 750°C, and eventually a reversed Pt segregation above 750°C, i.e. Co enrichment of the Pt 90Co 10(110) surface with respect to the bulk concentration. We find a clear correlation between the thermal deconstruction and the surface composition. The striking segregation reversal during temperature variation is attributed to the high excess value of the mixing enthalpy, which implies a structure-dominated segregation behavior.