PHASE-DIAGRAM AND PHASE-TRANSITIONS OF THE ADSORBATE SYSTEM S RU(0001) - A MONTE-CARLO STUDY OF A LATTICE-GAS MODEL/

We investigate the phase diagram and the critical properties of the ad sorbate system sulphur/ruthenium(0001) in the coverage region 0 < Thet a < 1/3 using Monte Carlo simulations of a lattice gas model on a tria ngular lattice. From experiments it is known that for low coverages an island phase appears in the phase diagram of this system at low tempe ratures. To capture this feature we include in our lattice gas model a weak third neighbour attraction in addition to the repulsive first an d second neighbour forces. The phase diagram obtained from simulations of this model is in very good agreement with the experimental phase d iagram. The critical properties of the lattice gas model are found to be compatible with the results of experiments on the system sulphur/ru thenium(0001). Finer details of the phase diagram, e. g. the location of tricritical points, which may be difficult to assess experimentally will also be discussed.