UNCONVENTIONAL ORIENTATIONAL GLASS TRANSITIONS IN SYMMETRICAL DIFLUOROTETRACHLOROETHANE

Frozen-in trans and gauche conformational disorder of symmetrical difl uorotetrachloroethane (DFTCE) produces the tendency of this plastic cr ystal to form an orientational glassy state on cooling below T(g) = 86 K. To throw light upon the freezing mechanism in DFTCE neutron diffra ction and Brillouin scattering as well as infrared and dielectric meas urements are employed. The overall picture of the results obtained can be characterized in terms of a classical glass-transition process wit h a relaxation frequency obeying the Vogel-Fulcher law. However, a mor e sophisticated picture of the freezing process is indicated by a quan titative analysis of the new Brillouin data, e.g. the temperature depe ndence of the sound velocity. The behaviour could be understood on the basis of a mode-coupling theory.