ORDERING IN LATTICE GASES WITH COMPETING INTERACTIONS ON INTERPENETRATING SQUARE SUBLATTICES - A MODEL OF 2-DIMENSIONAL SURFACE-COMPOUNDS ON SINGLE-CRYSTALS

We study a model of interacting lattice gases M and X on two interpene trating square sublattices with nearest neighbor attaction phi(MX) and next nearest neighbor repulsion phi(XX) in the X sublattice. By calcu lating ground-state energies of the relevant lattice gas phases LG, M, X, MX, c(2 x 2) X and c(2 x 2) M2X we obtain ground-state phase diagr ams for various values of R = phi(XX)/phi(MX). Standard Monte Carlo si mulation techniques are applied to investigate the lattice gas models at nonzero temperatures for the case R = -0.5. The simulations are per formed in the grand canonical ensemble with the chemical potentials mu (M), mu(X) and temperature T as independent variables. From adsorption isotherms theta(M)(mu(M), mu(X)) and theta(X)(mu(M), mu(X)) various p hase transitions of first and second order between different lattice g as phases are identified. We calculate phase diagrams for the case R = -0.5, which are compared to corresponding phase diagrams for R = 0 an d R = -1. Finally we discuss the applicability of such lattice gas mod els for the description of cosegregation processes.