Influence of phase transformation on the low temperature dielectric measurements in 1-cyanoadamantane.

Dielectric measurements have been performed on the supercooled plastic crystal phase of cyanoadamantane under isothermal and isochronal conditions in the temperature range [170-250 K], each experiment with a new sample. The measurements performed in the course of isotherms allowed us to emphasize the effects of the transformations close to the glass transition temperature leading to a change in the dynamic behavior of the compound. The temperature dependence of the dielectric relaxation times exhibits an Arrhenius evolution on the whole metastable domain and gives at the calorimetric glass transition temperature a value of 1 s, i.e., two orders of magnitude lower than the usual value [tau(T(g)) congruent with 100 s]. These results indicate that the Vogel-Fulcher-Tammann behavior observed with other techniques does not appear in the Brillouin zone center (q=0) and that the glass transition for this glass former is not associated with the freezing out of the tumbling motion of individual molecular dipoles but to the freezing out of fluctuations of an antiferroelectric local ordering.