Thermal Diffusion of Isotopes in Pure Liquids

A theoretical treatment is given of the recently discovered thermotransport effect in pure liquid metals. The isotope effect is related to the diffusive mass flow by a proportionality factor familiar from electrotransport. The mass flow is given primarily by the temperature dependence of the mean size or amount of density fluctuations in the liquid. Very little activation energy is required for diffusive motion, which takes place by cooperative position adjustments of neighbor atoms. The mean displacement length of the diffusing cluster is by an order of magnitude smaller than the effective cluster diameter. The liquid model suggested is coherent with models based on evidence from other experimental methods.