Tracer, jump, and chemical diffusion coefficients were determined by Monte Carlo simulations on a square lattice for the case of nearest-neighbor attraction and numerically equal next-nearest-neighbor repulsion. For this case only second-order θ vs μ/kT isotherms were found, indicating that the system was in a single phase at all coverages. <(δN)2>/<N> varied only very slightly with temperature at all coverages indicating balance between repulsive and attractive adsorbate-adsorbate interactions. Tracer and jump diffusion coefficients also showed only very slight temperature dependence, relative to the non-interactive case. The same was true for the chemical diffusion coefficient determined from the Kubo-Green equation as the product of jump diffusion coefficient and <N>/<(δN)2>, DKG. For JNN/kT < 1, there was reasonably good agreement between DKG and DF, the chemical diffusion coefficient determined by the fluctuation method. For JNN/kT > 1, DF decreased markedly with decreasing T. It was hypothesized that this behavior results from the freezing in of enough long-range order for this particular set of interactions to make the fluctuation results differ from the Kubo-Green ones. The latter were more microscopic, and it may be that the fluctuation results depend on probe size.

A Monte Carlo Study of Surface Diffusion Coefficients in the Presence of Adsorbate-Adsorbate Interactions. IV. Attractive Nearest-Neighbor and Repulsive Next-Nearest-Neighbor Interactions. C.Uebing, R.Gomer: The Journal of Chemical Physics, 1991, 95[10], 7648-52