Study of atomic jump processes in sodium crystals by quasi-elastic neutron scattering

The atomic jump processes involved in self-diffusion, in high-purity sodium single crystals were studied by means of quasi-elastic neutron scattering using a back-scattering spectrometer. In the temperature range between 323 K and the melting temperature (370.9 K) the dependence of the quasi-elastic line-width on the momentum transfer was measured in the major crystallographic and in various low-symmetry directions. The results are compared with calculations for various monovacancy and divacancy mechanisms of self-diffusion, incorporating spatial as well as temporal correlation effects of the atomic jumps on the basis of the encounter model and taking into account both nearest-neighbour and non-nearest-neighbour jumps. It is concluded that even close to the melting temperature self-diffusion in sodium occurs by nearest-neighbour jumps only. Satisfactory agreement between the diffusion coefficients obtained from our neutron-scattering data and the radiotracer experiments of Mundy (1971) can be achieved only if one assumes that both monovacancies and divacancies contribute to self-diffusion and that the divacancies migrate predominantly by 2n-4n transformations.