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 E-1 and E-2 for diffusion. A short-range repulsive interaction between particles in the nearest neighbour nonequivalent sites is considered, resulting in the modification of these energies by delta E, 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 Theta(c) which varies from Theta(c)=0.33 for delta E=0 to Theta(c)=0.5 for E-1 =E-2. 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. (C) 1999 Elsevier Science B.V. All rights reserved.