SURFACE-DIFFUSION IN THE PRESENCE OF PHASE-TRANSITIONS MONTE-CARLO STUDIES OF A SIMPLE LATTICE-GAS MODEL

A comparison of the Kubo-Green and fluctuation methods for determining surface diffusion coefficients of adsorbates was carried out by Monte Carlo simulations on a square lattice for attractive nearest and repu lsive next nearest neighbor interactions. Depending on R drop phi(NNN) /phi(NN) the lattice gas models show first- and second-order phase tra nsitions at low temperatures. In the presence of second-order phase tr ansitions (R = -2, R = -1) both methods agree, even in p(2 X 1) ordere d regions of the relevant phase diagrams. For systems with first-order phase transitions (R = -0.5, R = 0) we found a substantial remanent ' 'graininess'' of the lattice gas at temperatures well above T-c. This graininess represents early stages of first-order phase transitions be tween lattice gas phases of substantially different particle density. It is shown for the first time that such remanent graininess affects t he determination of the chemical surface diffusion coefficient, as dif fusion measurements may depend to some extent on the length scale of t he measuring technique.