DIFFUSION MECHANISM IN IRON-GROUP METAL CHALCOGENIDES

The diffusion mechanism of cations in iron-group metal chalcogenides with the NiAs type structure is discussed on the basis of microscopic jump frequencies from Mossbauer diffusional line broadening by the present authors and macroscopic diffusion coefficients by tracer method obtained by other investigators. For all the Fe1-xS samples except x = 0.003, the self-diffusion coefficients calculated from Mossbauer diffusional line broadening at high temperatures by assuming random walk in a two-dimensional pathway are in good agreement with the results obtained from tracer method, but at low temperatures the former shows larger values than the latter for larger deviation from stoichiometry due to the correlation of successive jump, based on the ordering of vacancies. The difference of diffusion coefficients between Mossbauer and tracer experiments for the composition of x = 0.003 with very small concentration of defects may be explained by considering the change of the predominant diffusion mechanism from two- to one-dimensional pathways and a small correlation factor. The diffusion mechanism of cations in other iron-group metal chalcogenides is discussed from the viewpoint of D(c)/D(a) ratio.