Diffusion of atoms in a dense adsorbed layer with a hexagonal structure

Abstract

The migration of an impurity atom over a honeycomb-type hexagonal lattice on a solid surface initiated by the diffusion of vacancies is theoretically studied. The case of small surface coverages of vacancies and impurity atoms is examined. The time dependence of the mean-square displacement at long time is demonstrated to be not very different from linear, whereas the spatial density distribution is close to a Gaussian profile, a result that makes it possible to introduce a diffusion coefficient. For the latter, an analytical expression is obtained, which differs from the product of the diffusion coefficient of vacancies and their relative concentration V_v only by a numerical factor. The dependence of the diffusion coefficient of the impurity atom on the ratio p of the frequency of its jumps to the frequency of jumps of vacancies is analyzed. In the kinetic mode, when the frequency of jumps of the impurity atom is small, the diffusion coefficient of the impurity is a linear function of p, while in the opposite case, a saturation takes place, making this coefficient independent of the frequency of jumps of the impurity atom.