Atomic motion in liquid sodium I. Diffusive motion

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Abstract
The time-of-flight technique has been used to measure the energy spectra of initially cold neutrons after scattering by liquid sodium at 115, 149 and 197 °c. Nearly-elastic peaks were observed, and have been compared with predictions from the model of Egelstaff and Schofield for atomic translational motion. Peak shapes and intensities are accurately fitted, and a disposable parameter c which describes the approach to the long-time behaviour given by Einstein's stochastic description has been determined at each temperature. The peak widths are in disagreement with the model of Singwi and Sjolander, but in accord with an extension of this model by Oskotskii. An essential feature of Oskotskii's extension is the concept of the collective diffusion of groups of atoms, and this type of motion is here found to account for 40% of the total diffusion coefficient of sodium. The parameter c of the Egelstaff-Schofield model may be interpreted as giving the mean number of atoms in a diffusing group; the present data yield a number of 30±1 atoms which is essentially constant over the temperature range studied. A comparison of these results with less extensive data for liquid lead and tin is made, and it is suggested that diffusive motion of groups of several tens of atoms could be a common feature of liquid metals. By contrast, a comparable analysis of data for liquid argon gives a number of atoms less than four for the size of any such group.

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