Surface composition of binary alloys

Abstract

A review is given of our present understanding of the laws which determine the surface composition of alloys in equilibrium. While the basic principles are fairly simple, their application is hampered by lack of numerical data on surface tensions and experimental verification of models was often unreliable because of unjustified neglect of the large contribution from subsurface atoms to, in particular, the peak intensity in Auger electron spectra. For perfect and regular solid solutions of atoms of equal size the “broken bond model” predicts a surface enriched with the element of lowest heat of atomization, the enrichment being strongest for faces of high coordinative unsaturation. In the case of slightly endothermic monophasic alloys this is accompanied by a smooth concentration gradient over several subsurface layers. For exothermic ordered alloys surface enrichment is counteracted by the energy associated with putting atoms in positions of the wrong sublattice in the interior. The subsurface is depleted of the element enriched in the outermost layer. In all monophasic alloys lattice strain effects have to be considered if atom sizes differ significantly. If such alloys are diluted, the strain effects tend to segregate solute atoms in the surface, but this phenomenon seems to be more pronounced for oversized than for undersized atoms. Also the broken bond model favours a surface enrichment with large atoms. Enrichment is, therefore, very large for alloys such as NiSn where these effects work together. This seems to be confirmed by new experimental data using inelastic ion scattering besides more conventional methods. For biphasic alloys a favourable arrangement is the “cherry model” where the phase of lower surface energy envelops the other phase. For all alloys the surface composition can be very strongly altered by adsorbing molecules, the element with highest heat of adsorption segregating in the surface. A brief review of the modern experimental methods is given and the geometrical and electronic effects of surface composition on chemisorption and catalysis are illustrated with one example.