Tracer-diffusion measurements were made of 45Ca and 22Na co-diffusion into pure NaCl as a function of temperature, initial Ca concentration, and annealing time. It was found that the penetration profiles deviated markedly from the Gaussian dependence which was expected for diffusion from a thin source. The curvature was attributed to the fact that each divalent Ca ion introduced an extra ‘extrinsic’ vacancy into the diffusion zone in order to maintain charge neutrality. The effect entered, to first order, into measurements of the isotope effect in ionic crystals, and could result in large errors in the measured isotope effect for even small deviations from a Gaussian profile. The diffusivity of 45Ca, at temperatures ranging from 575 to 775C, could be described by:

D (cm2/s) = 0.23 exp[-1.61(eV)/kT]

J.L.Mitchell, D.Lazarus: Physical Review B, 1975, 12[2], 734-52