PHASE-DIAGRAMS FOR SURFACE ALLOYS

We discuss surface alloy phases and their stability based on surface p hase diagrams constructed from the surface energy as a function of the surface composition. We show that in the simplest cases of pseudomorp hic overlayers there are four generic classes of systems, characterize d by the sign of the heat of segregation from the bulk and the sign of the excess interactions between the atoms in the surface (the surface mixing energy). We also consider the more complicated cases a with or dered surface phases, nonpseudomorphic overlayers, second layer segreg ation, and multilayers. The discussion is based on density-functional calculations using the coherent-potential approximation and on effecti ve-medium theory. We give self-consistent density-functional results f or the segregation energy and surface mixing energy for all combinatio ns of the transition and noble metals. Finally we discuss in detail th e cases Ag/Cu(100), Pt/Cu(111), Ag/Pt(111), Co/Cu(111), Fe/Cu(111), an d Pd/Cu(110) in connection with available experimental results.