Sodium self-diffusion and the isotope effect

Abstract

Measurements have been made of the variation of the self-diffusion coefficient of solid sodium with the mass of the diffusing atom using ²²Na and ²⁴Na as tracers. The diffusion of ²²Na in sodium has been measured over the temperature range 0–97°c and the data accurately fit the Arrhenius relation: Isotope effect measurements have been made over the temperature range 40–97°c. Between 40 and 90°c [(D ²²/D ²⁴)−1] − 0·0163±0·0005. The results indicate that diffusion occurs by a relaxed vacancy mechanism. Close to the melting point there is a reduction of the isotope effect and an enhancement of the diffusion coefficient. It is postulated that this enhancement is due to homogeneously distributed molten regions in the sodium caused by the presence of potassium impurity. On this basis it is inferred that the isotope effect for liquid sodium is close to zero.