CO-SEGREGATION AT THE SURFACE OF PB-BI-NI ALLOYS - COMBINED AB-INITIOAND MONTE-CARLO STUDY

A recent study of a Pb-Bi-Ni alloy containing 5 at.% Pi and 0.04 at.% Ni reported a strong co-segregation of Pi and Ni at the alloy surface. We have performed ab initio calculations of the segregation profiles at the (111), (100) and (110) surfaces of random Pb95Bi5 alloys by mea ns of the coherent potential approximation and the tight-binding linea r muffin-tin-orbitals method. We have found the segregation profiles t o be oscillatory (this effect is most pronounced for the (111) surface ) with a strong preference for Pi to segregate to the first atom layer and depletion of Pi in the subsurface atom layer. The energetic origi n of the oscillatory segregation is discussed in terms of the Connolly -Williams effective cluster interactions. In the ternary Pb95Bi5 + Ni alloy we have also found a tendency for Ni to segregate to the subsurf ace atom layer due its strong interaction with Bi, which is present at high concentrations relative to the bulk in both the first and third atom layers of the (111) surface. In order to include relaxation effec ts, we have performed Monte Carlo simulations, employing Finnis-Sincla ir-type empirical many-body potentials, and computed the segregation p rofiles at the (111) surface of Pb95Bi5 and Pb95Bi5 + Ni alloys. For P b-Bi alloys, the concentration profiles have also been found to be osc illatory, in fair agreement with results of the ab initio calculations . The calculations on Pb-Bi-Ni show strong segregation of Ni to the su bsurface atom layer, accompanied by co-segregation of Pi to several of the outermost atom layers.