Monte Carlo study of surface phase transitions in lattice gases on interpenetrating square sublattices

Abstract

Monte Carlo simulations were performed using a lattice gas model of two interpenetrating square sublattices, which each may accommodate one type of adspecies M or X. The simulations were carried out in the grand canonical ensemble, i.e., with the chemical potentials μ M, μ X and temperature T as independent variables. Pairwise nearests-neighbor attractions φ MX were taken into account. Adsoption isotherms θ M(μ M, μ X) indi θ X(μ M, μ X) indicate first-order phase transitions at low temperatures separating high- and low-coverage phases, respectively. Depending on μ M, μ X and T the stoichiometries of these phases show significant deviations from the ideal composition MX. At high temperatures the miscibility gaps disappear and continuous changes of the lattice gas compositions were found upon varying the system parameters. The results are applied to the findings of recent investigations of surface cossegregation phenomena showing the existence of two-dimensional surface compounds MX on bcc (100) surfaces.