Analysis of the Arrhenius shape of adatom diffusion coefficient in a surface model with two energy barriers

The effective energy barrier for diffusion, E-a, has been previously determined as a function of coverage and temperature within a two-barrier model of diffusion, assuming an Arrhenius form of chemical diffusion coefficient. To determine the diffusion coefficient at a given temperature and coverage, we used previously published results based on the lattice gas model of non-interacting adatoms. The presence of two non-equivalent sites for adsorption and diffusion of atoms on a surface (with energy barriers E-1, E-2) results in substantial changes to E-a within coverage Theta, with a step-like shape having a value close to Theta=1/3. The temperature dependence of E-a shows a slight minimum, and the Arrhenius shape (with constant E-a) seems to be good approximation of the relatively small differences in energy barriers E-1, E-2. We obtain considerable deviation from such an Arrhenius behaviour of diffusion when there is a large difference in the energy barriers. (C) 1999 Elsevier Science B.V. All rights reserved.