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

Chemical ordering and clustering instabilities in alloys are governed by the Fourier transform of the effective pair interactions, V (k). We make use of a second-order-expansion formalism, based upon embedded-atom-method interatomic potentials, to calculate chemical and elastic contributions to V (k) for monolayer surface alloys on single-crystal substrates. It is demonstrated 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) ∝ −|k| for small wavevectors. As a consequence, the global minimum in V (k) always occurs at finite k, and therefore compositional instabilities in ultrathin surface-alloy films are generally of an ordering (as opposed to clustering, k = 0) type.