1ST-ORDER AND 2ND-ORDER PHASE-TRANSITIONS IN A SIMPLE LATTICE-GAS MODEL

We investigate the phase diagram and the critical properties of the adsorbate system sulphur/ruthenium(0001) in the coverage region 1/4 < theta < 1/3 by means of Monte Carlo simulations of a simple lattice gas model on a triangular lattice. The model contains only repulsive nearest- and next-nearest-neighbour interactions. Combining results obtained by using both Glauber and Kawasaki kinetics in the simulations, we identify two tricritical points, three coexistence regions and an eutectic point in the phase diagram of the system. Our results agree with the findings of recent experimental work on S/Ru(0001). Furthermore, we are able to add certain details to the experimentally observed phase diagram.