Equilibrium surface segregation of interstitials on bcc(001) surfaces: A lattice-gas approach.

Abstract

A lattice-gas model for the description of equilibrium surface segregation of interstitials on (001)-oriented body-centered-cubic surfaces is introduced. Assuming realistic nearest and more distant pairwise interactions between adjacent M-X and X-X atoms, the model is well suited to reproduce the experimentally observed c(2\ifmmode\times\else\texttimes\fi{}2) ordering of segregated nitrogen and carbon on \ensuremath{\alpha}-Fe(001). With increasing range of the repulsive X-X interactions ${\mathit{cphi}}_{\mathit{X}\mathit{X}}$(d), different orderings such as (2\ifmmode\times\else\texttimes\fi{}1), (2\ifmmode\times\else\texttimes\fi{}2), and c(4\ifmmode\times\else\texttimes\fi{}2) occur at \ensuremath{\theta}=1/2 and \ensuremath{\theta}=1/4, 3) / 4 . The coverage dependence of the excess Gibbs free energy of segregation is discussed for the present lattice-gas model.