Surface diffusion on a square lattice with two non-equivalent sites

We investigate surface diffusion in a system of particles adsorbed on a square lattice with two non-equivalent sites. Analytical expression for the chemical diffusion coefficient have been derived in case of strong inhomogeneity of the lattice. It is shown that the character of the particle migration depends substantially on the relation between the jump frequencies. When the frequencies differ insignificantly, the particle diffusion proceeds by single uncorrelated jumps of particles between the nearest neighbor lattice sites. In the opposite case, when the jump frequencies differ considerably (slow and fast jumps), the transfer of the particles over the surface proceeds by the pairs of consecutive jumps: a slow jump is followed by a fast jump. There are two types of such jump pairs. We have calculated the coverage dependencies of the tracer, jump and chemical diffusion coefficients for the Langmuir lattice gas for some representative temperatures using analytical and numerical approaches. The coincidence of the data obtained by the two quite different methods is rather good.