Low-energy ion scattering (LEIS) was used to study the surface composition of a alloy surface as a Pt 80
Fe 20 (111) function of temperature. From 700 K onwards, high enough for sufficient atomic mobility to attain equilibrium, the surface consists of nearly pure Pt. These results agree completely with the results from LEED I/V analysis by Beccat et al. [ Surf. Sci. 238, 105 (1990) ] , who also evidenced a monotonic concentration proÐle over the three top layers, slight relaxations between those layers and a (2 Â 2) superstructure caused by an ordered atomic arrangement in the second layer.
Simple expressions for the entropy changes upon segregation in ordered alloys combined with constant pairwise bond energies clearly appear to be inadequate to describe the observed experimental facts. Even when combined with Monte Carlo simulations, the constant bond energies fail to correctly reproduce the results. Only when the Monte Carlo method is combined with the more reÐned (modiÐed) embedded atom method (MEAM) for energy description of an alloy is satisfactory agreement with experiment obtained.