EQUILIBRIUM SURFACE SEGREGATION OF INTERSTITIALS ON BCC(001) SURFACES- A LATTICE-GAS APPROACH

A lattice-gas model for the description of equilibrium surface segrega tion of interstitials on (001)-oriented body-centered-cubic surfaces i s introduced. Assuming realistic nearest and more distant pairwise int eractions between adjacent M-X and X-X atoms, the model is well suited to reproduce the experimentally observed c(2 x 2) ordering of segrega ted nitrogen and carbon on alpha-Fe(001). With increasing range of the repulsive X-X interactions phi XX(d), different orderings such as (2 x 1), (2 x 2), and c(4 x 2) occur at theta = 1/2 and theta = 1/4, 3/4. The coverage dependence of the excess Gibbs free energy of segregatio n is discussed for the present lattice-gas model.