Diffusion on a face-centred cubic (111) surface in the presence of two non-equivalent adsorption sites

The interrelation between the surface symmetry and diffusion of adsorbed atoms is studied theoretically on a model face-centred cubic (fcc) (111) surface within the lattice-gas approximation. Diffusion proceeds through two non-equivalent sites on the fcc(111) surface. Short-range repulsive interaction between particles in the nearest-neighbour non-equivalent sites is considered. This repulsion leads to a modification of the occupation numbers of both kinds of site and to changes in the energy barrier height for diffusion. The chemical surface diffusion coefficient, D, has been found to be strongly dependent on coverage, Θ, the interaction strength between adatoms, and the difference between the activation energies, ΔE. In the case of a small ΔE value, the diffusion coefficient D increases with Θ, as was expected for repulsive interaction. In our model the increase of D at Θ=0.5 is reduced for higher values of ΔE and a local maximum of D(Θ) is observed.