Metastability of the segregation-reversal composition profile at the ${\mathrm{Pt}}_{\mathrm{x}}$${\mathrm{Ni}}_{1\mathrm{-}\mathrm{x}}$(110) surface: A low-energy electron-diffraction study

A low-energy electron-diffraction study of the surface segregation at ${\mathrm{Pt}}_{10}$ ${\mathrm{Ni}}_{90}$(110) is presented. The Pt concentrations in the first and second layers are, respectively, 6 and 52 at. %. The corresponding interlayer spacings differ from the bulk values by -4.5% and +3.6%. This oscillating behavior is similar to that reported previously for ${\mathrm{Pt}}_{50}$ ${\mathrm{Ni}}_{50}$(110), the most spectacular result being that the segregation is also reversed with respect to the (111) face. Our results are discussed in the light of theoretical models which predict Pt segregation for Ni-rich alloys, Ni segregation for Pt-rich alloys, and a metastable situation in the range 30–60 at. %. We also discuss the influence, on chemical reactivity, of (i) the segregation reversal, (ii) the strong Pt enrichment in the second layer, and (iii) the two-dimensional ordering tendency.