The energetics of surface-alloy formation: an embedded-atom-method, second-order-expansion study

Chemical ordering and clustering instabilities in alloys are governed by th e Fourier transform of the effective pair interactions, V(k). We make use o f a second-order-expansion formalism, based upon embedded-atom-method inter atomic potentials, to calculate chemical and elastic contributions to V(k) for monolayer surface alloys on single-crystal substrates. It is demonstrat ed that the elastic contribution to V(k) is characterized by a finite slope at the origin, consistent with continuum models which predict that V(k) in finity -\ k \ for small wavevectors. As a consequence, the global minimum i n V(lc) always occurs at finite k, and therefore compositional instabilitie s in ultrathin surface-alloy films are generally of an ordering las opposed to clustering, k = 0) type.