Phase transitions in solid methanol

Neutron powder diffraction patterns were measured for the ordered α-phase and the disordered β-phase of deuterated methanol. The structure of the α-phase at 160 K is in agreement with that found earlier at 15K. A complete refinement of the structure of the β-phase at 170 K was also carried out. The space groups are α-P2₁2₁2₁ and β-Cmcm. For the disordered phase, the thermal parameters indicate that the molecules are localized rather than being in free rotation. A transformation matrix was found that relates the unit cells of the two phases. The transition involves mainly the large-angle rotation of molecules in a plane. In a second experiment, the α- to β-phase transition in both deuterated and undeuterated solid methanol was examined using Raman spectroscopy and a metastable phase was produced, for the undeutered sample, by rapid quenching through the phase transition. Only two modes of the methyl groups in this metastable phase differ from the internal modes of the stable α-phase.