By means of Monte Carlo computer simulation, the extent of validity of the so-called impurity form of the tracer correlation factor in a model of a stoichiometric binary alloy (Ising magnet analogue) was explored. Above the order-disorder temperature, the impurity form of the correlation factor appeared to be valid. Below the order-disorder temperature, there was a deviation between the actual tracer correlation factor and that predicted by means of the impurity form. This deviation increased substantially as the temperature decreased. These findings implied that the measured isotope effect in ordered alloys may not in fact be directly related to the usual tracer correlation factor.

Computer Simulation of the Diffusion Isotope Effect in a Binary Ordered Alloy. L.Zhang, W.A.Oates, G.E.Murch: Philosophical Magazine A, 1989, 59[1], 171-7