Neutron-scattering study of metallic sodium

A neutron elastic- and inelastic-scattering study has been performed on single crystals of metallic sodium over a temperature range of 10 to 300 K. On cooling the virgin sample, the incubation time to transform from the bcc structure to the low-temperature structure was found to be more than 2 h at 38 K. The full width at half maximum (FWHM) of the (110${)}_{\mathrm{bcc}}$ Bragg reflection suddenly increased from 0.3° to 0.73°, due to some deformation introduced by the nucleation of the low-temperature structure. In relation to the deformation, strong extra-diffuse scattering was observed around the (110${)}_{\mathrm{bcc}}$ Bragg reflection in addition to thermal diffuse scattering. On warming through the transformation, the FWHM gradually recovered but was larger than that of the virgin state. Apparently some deformation still remained in the bcc structure even though the low-temperature structure disappeared. It was estimated that the cluster size of the low-temperature phase is more than 3.2 nm along the c axis from analyzing the Huang-type diffuse scattering. On cooling again, the phase transition did not take place until 10 K and a waiting time of at least 20 min at that temperature was needed before the transformation took place. No drastic change in the ${\mathrm{TA}}_1$[110] phonon-dispersion branch down to 38 K was observed, though the phonon energies for q≥0.25 decreased slightly with decreasing temperature.