O NI(111) - LATERAL INTERACTIONS AND BINDING-ENERGY DIFFERENCE BETWEEN FCC AND HCP SITES/

Based on low-energy electron diffraction structural investigations of ordered and disordered phases of O/Ni(111) that show that occupation o f both fcc and hcp sites can be forced either by thermal activation or by coverage, the lateral interactions and the binding-energy differen ce of fcc and hcp sites are redetermined for this system by simulating the phase diagram with Monte Carlo simulations, concentrating on the coverage range between 0.25 and 0.33 ML. From a comparison of the temp erature dependence of the occupation probability of fcc and hcp sites in experiment and simulations, the difference in binding energy betwee n fcc and hcp sites is determined to be 46 meV. Using a minimum set of five paint ise lateral interactions, the experimental phase diagram i n the investigated coverage range is reproduced in detail. Even the co mplex diffraction patterns experimentally observed in the domain-wall phase are well reproduced in the simulations. We show in particular th at this phase is only stabilized by entropy. Critical properties of th e p(2 X 2) order-disorder transition are discussed briefly. [S0163-182 9(97)03540-6].