Diffusion kinetics and isotope effects for atomic migration via divacancies and triple defects in the CsCl (B2) structure

Expressions for the diffusion coefficients and correlation factors for diffusion by means of divacancies in ionic compounds with the CsCl (B2) structure are calculated. Values for the correlation factors as a function of the ratio of the defect-atom exchange frequencies of both components are tabulated. The correlation factors are obtained in explicit form. From this it turns out that a measurement of isotope effects would yield (in principle) the values of the correlation factors. It is demonstrated that, in a measurement of diffusion coefficients, differences in jump frequencies and activation energies between both components are decreased by correlation effects. For intermetallic compounds with the triple-defect B2 structure, atomic migration via triple defects has been proposed (Stolwijk, Van Gend and Bakker 1980). Expressions for the diffusion coefficients of both components via this defect are presented and it is shown how values for the correlation factors can be deduced from the divacancy case. It is demonstrated that a measurement of the isotope effect would give (in principle) the values of the correlation factors as defined in the present paper. Numerical values for such an isotope effect measurement are predicted. The diffusion coefficients for correlated migration of divacancies and triple defects themselves are also calculated.