SURFACE SEGREGATION IN PTRH ALLOYS REVISITED IN THE FRAMEWORK OF THE TIGHT-BINDING ISING-MODEL

The recently developed tight-binding Ising model (TBIM) is used to interpret the puzzling experimental results on surface segregation in PtRh alloys. More precisely, both the nature of the segregant element (Pt) and the anomalous increase with temperature (up to 1000 K) of the surface enrichment have required some authors to invoke a generally neglected effect: the vibrational entropy. We show here that this term is indeed negligible and that the TBIM, which is derived from electronic structure, accounts for the experimental observations (Pt surface enrichment, oscillating concentration profile). As for the apparent increase in surface segregation with temperature, it is attributed to kinetics limitations which prevent from reaching the equilibrium. © 1990.