Self-diffusion mechanisms in sodium

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Abstract
The vacancy cannot bc ruled out as an important self-diffusion mechanism in sodium, as has recently been suggested, on the basis of the large isotope effect calculated according to the formalism of the reaction rate theory. Experiment shows that several mechanisms, all with small individual isotope effects, contribute to diffusion over a wide temperature range. Since one of these mechanisms is almost certainly to be identified with the vacancy, theory is still faced with explaining a small isotope effect for the vacancy mechanism. This difficulty is resolved by showing that the large calculated isotope effect is a characteristic feature of the rate theory, but not of the alternative dynamical theory of diffusion. Although no definitive assignment of mechanisms can yet be made, the available evidence is consistent with a picture involving two types of interstitial (of nearly equal energy) and vacancies.