Diffusion correlation effects and the isotope effect in intermetallic compounds

There has been considerable discussion in the literature about the various roles and meanings of the diffusion correlation factors in ordered alloys. In this paper diffusion in a two-dimensional stoichiometric ordered alloy m odel is considered. Using a combination of Monte Carlo simulation and analy tical development we consider how the correlation factor for diffusion, as would be found from an isotope effect experiment, is related to the actual correlation factor (all jumps included) and the correlation factor for effe ctive diffusion by the six-jump-cycle mechanism. It is shown that the appar ent correlation factor from an isotope effect experiment can deviate very g reatly from the actual correlation factor but extrapolates precisely to tha t for the six jump cycle when the long range order parameter approaches uni ty. This clarifies the relationships between the various correlation factor s in ordered alloys where intersublattice jumps predominate.