A new interpretation of tracer diffusion in Co-Ga

Tracer diffusion of Co and Ga in Co-Ga has formerly been described primaril y in terms of a triple-defect mechanism based on a divacancy mechanism and the existence of triple-defect disorder. Using the interaction energies emp loyed to describe the partitioning of the vacancies and atoms in Co-Ga by W aegemaekers and Van Ommen (1982, J. Phys. F, 12, 2829) we have performed Mo nte Carlo simulations of the tracer diffusivities in Co-Ga. We make use of a general isolated vacancy diffusion model with nearest-neighbour jumps wit h no explicit or implicit binding between the vacancies that others have su ggested previously. The model is fully consistent with triple-defect disord er in Co-Ga. We show that the tracer diffusion data can be well described i n Co-Ga without the need for the triple-defect mechanism. It is found that Ga makes the equivalent of a next-nearest-neighbour jump by jumping via the Co sublattice where it has a very short residence time whereas the triple- defect mechanism requires a formal next-nearest-neighbour jump for Ga. It i s commented that tracer diffusion measurements alone probably cannot resolv e this difference.