CLUSTER EXPANSIONS AT ALLOY SURFACES - FORMALISM AND APPLICATION TO SEGREGATION IN NI-CU

A formalism is described to calculate concentration-independent effect ive cluster interactions at an alloy surface in the ''unrestricted'' s cheme. Although this approach is generally valid, a specific implement ation is discussed in the context of the tight-binding approximation i n conjunction with the direct configurational averaging method. This t echnique is applied to a study of segregation at the (100) surface of NicCu1-c alloys. Monte Carlo simulations show a strong tendency for Cu to enrich the surface at all temperatures and a monotonic approach of the equilibrium segregation profile to the bulk composition. The latt er point has been a source of some controversy because a number of rec ent calculations, using widely different techniques, produce conflicti ng results. A critical discussion is given df the relative merits of t he various methodologies.