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Abstract:
The effective energy barrier for diffusion, Ea, 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 E1, E2) results in substantial changes to Ea within coverage Θ, with a step-like shape having a value close to Θ=1/3. The temperature dependence of Ea shows a slight minimum, and the Arrhenius shape (with constant Ea) seems to be good approximation of the relatively small differences in energy barriers E1, E2. We obtain considerable deviation from such an Arrhenius behaviour of diffusion when there is a large difference in the energy barriers.
