MONTE-CARLO STUDY OF ORDERING IN LATTICE GASES ON INTERPENETRATING SQUARE SUBLATTICES

Monte Carlo simulations were performed using a lattice-gas model of tw o interpenetrating square sublattices, each of which may accommodate o ne type of adsorbed species M or X. The simulations were carried out i n the grand canonical ensemble, i.e., with the chemical potentials mu( M) and mu(X) and temperature T as independent variables. Pairwise near est-neighbor attractions phi(MX) and next-nearest-neighbor repulsion p hi(XX) were taken into account. It is shown that this model is well su ited to describe the formation of two-dimensional surface compounds M2 X, e.g., Cr2X and (CrxFe1-x)2S recently observed on Fe-15 wt, % Cr-S ( 100) surfaces. Moreover this model predicts a variety of different sec ond- and first-order surface phase transitions which as of yet have no t been observed experimentally.