Structural correlations in disordered matter: An experimental separation of orientational and positional contributions

The structures of the liquid, glass (i.e., amorphous), rotator-phase (RP) crystal, orientational glass (OG), and the stable (monoclinic) crystal phases of ethanol are investigated by means of neutron and x-ray diffraction. The RP crystal and OG phases show an intermediate kind of disorder between that of the fully disordered glass and liquid phases and that of the stable crystal. The Bragg pattern of the RP and OG crystals provides information about the time-averaged distribution of single-molecule orientations, and a contribution comprising intermolecular correlations is isolated from the diffuse intensity. Finally, the $\mathrm{RP}\to \mathrm{OG}$ rotational freezing transition is analyzed in detail and shown to constitute a physical realization of the glass transition found in hard-needle models.