A MONTE-CARLO STUDY OF SURFACE-DIFFUSION COEFFICIENTS IN THE PRESENCE OF ADSORBATE ADSORBATE INTERACTIONS .3. REPULSIVE NEAREST-NEIGHBOR AND ATTRACTIVE NEXT-NEAREST-NEIGHBOR INTERACTIONS

Tracer, jump, and chemical diffusion coefficients D*, D(j), and D, respectively, were determined on a 50 X 50 square array of fixed sites by the fluctuation and Kubo-Green methods for the case of nearest-neighbor repulsions and numerically equal next-nearest-neighbor attractions. Both methods give the same D values except at theta = 0.5 and T/T(c) < 0.9, where c(2 X 2) dominates and simulations are extremely sensitive to very small changes in the small amount of disorder. Activation energies are, in all cases, controlled by next-nearest-neighbor attraction, i.e., are positive relative to the noninteracting case and peak at theta = 0.5. However, D has a maximum at theta = 0.5 because of a deep minimum in <(delta-N)2>/<N>. D* and D(j) are very similar to each other and have a minimum at theta = 0.5 and low T, because of the maxima in activation energies at this coverage, caused by the necessity to deform the c(2 X 2) structure.