Surface segregation in simple metal alloys: An electronic theory

Abstract

Surface segregation in simple-metal random binary alloys is studied via an electronic theory based on local ionic pseudopotentials and a linear-electron-response model appropriate for semi-infinite systems. Segregation of the larger species ions to the surface layer and (in most cases) a nonmonotonic layer concentration profile are predicted. The segregation of the larger species ions to the surface layer is driven by single-particle terms (Hartree-energy terms) in the total-energy expansion. These single-particle energy terms are independent of coordination number and relative positions of the ions but depend on the position of the surface layer relative to the inhomogeneous zeroth-order electron density at the metal surface, thus giving rise to crystal-face specificity. The concentration in deeper layers is determined primarily by effective interionic interactions. The electronic theory is compared with a nearest-neighbor pair-bond model, and it is concluded that the pair-bond model is not applicable to surface segregation in simple-metal alloys. The alloys considered in this paper are composed of the alkali metals K, Rb, and Cs. Concentration profiles as functions of temperature are presented for the (100) and (110) surfaces.