Surface-sandwich segregation in Pt-Ni and Ag-Ni alloys: Two different physical origins for the same phenomenon

We show that a simple mean-field approach, including simultaneously a correct size effect (treated atomistically), a surface-tension term and the alloy effect, is sufficient to account for the surface-``sandwich'' segregation observed on the (111) faces of Pt-Ni and very dilute Ag:Ni alloys. We show that this phenomenon does not have the same physical origin in both cases.