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Abstract

We calculated the surface diffusion coefficient D as a function of coverage in the quasi-chemical approximation of a system of particles on a square lattice with two non-equivalent binding sites characterized by different activation energies E₁ and E₂ for diffusion. A short-range repulsive interaction between particles in the nearest neighbour non-equivalent sites is considered, resulting in the modification of these energies by δE. The thermodynamic and kinetic parts of D have been calculated separately, in order to estimate their contribution to the diffusivity at different concentrations. It has been found that D exhibits a step-like increase at a critical coverage Θ_c which varies from Θ_c=0.33 for δE=0 to Θ_c=0.5 for E₁=E₂. The origin of the concentration dependence of D is discussed in terms of the repulsion strength between particles and their jump-rate through the sites existing on the surface.