A new interpretation of tracer diffusion in Co-Ga

Tracer diffusion of Co and Ga in Co-Ga has formerly been described primarily in terms of a triple-defect mechanism based on a divacancy mechanism and the existence of triple-defect disorder. Using the interaction energies employed to describe the partitioning of the vacancies and atoms in Co-Ga by Waegemaekers and Van Ommen (1982, J. Phys. F, 12, 2829) we have performed Monte Carlo simulations of the tracer diffusivities in Co-Ga. We make use of a general isolated vacancy diffusion model with nearest-neighbour jumps with no explicit or implicit binding between the vacancies that others have suggested previously. The model is fully consistent with triple-defect disorder in Co-Ga. We show that the tracer diffusion data can be well described in 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 is commented that tracer diffusion measurements alone probably cannot resolve this difference.