Theory of surface enrichment and surface disorder effects in binary alloys

Abstract

Cooperative effects at binary (AB) surfaces are investigated. In general, the concentration of B-atoms at the surface will differ from the bulk (‘surface enrichment’). But it may also occur that phase separation occurs at the surface of alloys which are miscible in the bulk (‘surface segregation’). These effects are possible even if the bulk displays order-disorder phenomena. On the other hand, surface order-disorder phenomena may occur rather than surface segregation effects. We study these effects by phenomenological theories (using the concept of ‘extrapolation lengths’), microscopic (Ising-) models and Monte Carlo computations, similar to previous studies of magnetic surfaces by Binder and Hohenberg. We obtain the concentration ‘profiles’ near the surface of bulk samples, thin films and small grains in several of these cases, and find that these ‘profiles’ are quite smooth and monotonie. Attention is paid to treat the effects of concentration fluctuations, particularly near the respective critical points of ordering or phase separation. It is shown that the phenomenological theory cannot account for the correct scaling properties in the general case, even if one allows for an arbitrary temperature dependence of the extrapolation length. But from the computer simulation detailed numerical results are obtained for the surface enrichment as a function of temperature and bulk concentration. The typical order of magnitude of this surface enrichment is 0.1. A simple model including the effects of surface roughness is also treated. Possible experimental applications (catalysis phenomena, etc.) are briefly mentioned.