A four-frequency diffusion model was considered that posited the diffusion of paired and unpaired atoms (or small molecular species) in an otherwise empty lattice. The model was useful for describing tracer and collective diffusion of interstitials in dilute interstitial solid solutions and of adsorbed species on surfaces. Diffusion kinetics of this model were treated analytically by McKee, LeClaire and Okamura and Allnatt. Monte Carlo computer simulation was used to calculate, for the first time, the correlation factors and the linear solute diffusion enhancement factor. Absolute values of the tracer correlation factors and the linear enhancement factors were in excellent agreement with the results of the theory of Okamura and Allnatt, but only in semi-quantitative agreement with results of the other theory. A general argument was also presented to show that the chemical diffusion coefficient did not exhibit correlation effects. This was verified by computer simulation.

Diffusion Kinetics of Isolated and Paired Species: a Monte Carlo Study. I.V.Belova, M.J.Brown, A.R.Allnatt, G.E.Murch: Philosophical Magazine, 2005, 85[24], 2787-98