MONTE-CARLO STUDIES OF ADSORBED MONOLAYERS - LATTICE-GAS MODELS WITH TRANSLATIONAL DEGREES OF FREEDOM

Standard lattice-gas models for the description of the phase behavior of adsorbed monolayers are generalized to ''elastic lattice gases'' wh ich allow for translational degrees of freedom of the adsorbate atoms but have the substrate lattice structure built into the adsorbate-adso rbate interaction. For such models, we derive a simple and efficient g rand-canonical Monte Carlo algorithm, which treats the occupied and em pty sites in precisely the same way. Using this method, we calculate t he phase diagram of a simple model for the adsorption of hydrogen on p alladium (100); this model includes only pairwise interactions and exh ibits an ordered c(2x2) structure. For our choice of parameters, we fi nd only a rather small influence of the translational degrees of freed om on the phase diagram. In particular, the observed asymmetry, albeit clearly present, is quite weak. Finite-size scaling reveals that the second-order phase transition between c(2x2) and the disordered phase is Ising-like, i.e., the elastic degrees of freedom do not change the universality class.