Thermodynamics of adatoms diffusing on a surface with two different sites: A new type of phase transition

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, E-1 and E-2. 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 E-1 - E-2. No lateral interactions are considered. Depending on the sign of E-1 - E-2, 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 E-1 - E-2 --> +/-infinity. 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. (C) 1997 Elsevier Science B.V.