Isotope effect of diffusion of 22Na+24Na+ in NaCl,KCl and KBr single crystals

Accurate measurements of the diffusion coefficient and isotope effect for diffusion of Na⁺ have been carried out in the intrinsic range of NaCl, KCl and KBr single crystals. The technique which led to the more reliable results is described. This is based on the simultaneous use of the differences between the half-life times and the energy spectra of γ-radiation of the isotopes ²²Na and ²⁴Na after diffusion into two adjacent samples. In NaCl, the isotope effect is found to vary very little with temperature around the mean value 0·75, while the isotope effect decreases from 0·68 at 772°C down to 0·44 at 574°C in KCl, and from 0·64 at 693°C down to 0·52 at 601°C in KBr. Comparison of the vacancy pair contributions to the self-diffusion of Na⁺ and Cl^?1] in NaCl is attempted. Comparison of the isotope effect in the different matrixes shows an influence of the relative ion sizes in agreement with the theory of Le Claire2].