Structures of sodium metal

Abstract

The martensitic transformation in sodium metal has been studied using neutron-diffraction and neutron-inelastic-scattering techniques. In two separate experiments the transformation was observed on cooling near 32 K. Measurements of the diffuse scattering, diffraction linewidths, quasielastic scattering, and the temperature dependence of the ${\mathrm{\ensuremath{\Sigma}}}_{4}$[hh0] phonon energies above the transformation show no evidence of transformation precursors. The sodium martensite appears as 24 rhombohedral variants, four about each bcc (110), with reflections from their basal planes at (1.018, 0.92, \ifmmode\pm\else\textpm\fi{}0.06), (0.92, 1.018, \ifmmode\pm\else\textpm\fi{}0.06), and equivalent points. The layer stacking order of the variants is fixed by their relationship to the host bcc material. The crystallography of the low-temperature phase is shown to be a complex mixture of stacking-fault-affected rhombohedral polytypes of a particular ``almost-hexagonal'' structure. These form a ladder of structures connected, one to another, by stacking faults. As the martensite is warmed and before the complete reversion to a bcc structure, the relative fraction of the different martensite phases changes. Near 55 K, the longest-period polytypes are the most stable.