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Abstract:
The chemical surface diffusion coefficient D has been determined for a lattice-gas model where diffusion proceeds through two non-equivalent lattice sites with different activation energies, E1 and E2. By calculating the thermodynamic factor as well as jump diffusion rate we obtain the coverage dependence of D as a function of the energy difference E1-E2. No lateral interactions are considered. Depending on the sign of E1-E2, the diffusion coefficient shows either a maximum or a step-like increase at a particular coverage, both of which develop into discontinuities in the limits of either E1-E2 → ± ∞. In the case of the step-like behaviour of D, the variation of D can be classified as being due to a kind of diffuse phase transition between a low and a high diffusivity state, which in the limits is shown to be of first order.
