Surface diffusion on a lattice with two non-equivalent adsorption sites: repulsive interactions

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 Θc which varies from Θc=0.33 for δE=0 to Θc=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