Partial ordering of supercooled liquid ethanol into a rotator-phase crystal as an entropy-driven transition

A rotator-phase (plastic) crystal phase is formed upon annealing supercooled liquid ethanol several degrees above the glass transition temperature. The mechanism driving the transformation is shown to be of entropic nature as monitored by the excess linewidth of the neutron quasielastic spectra of the rotator-phase crystal with respect to the supercooled liquid. The found phenomenon constitutes a physical realization of those already known to occur in hard-core systems which show ``pure'' entropy-driven ordering transitions.