Supercooled liquids and plastically crystalline phases: Indications for a similar manifestation of a crossover in the evolution of the dielectric spectra

We compare the susceptibility spectra (10−6Hz–1012Hz) of glass forming liquids and plastically crystalline (PC) phases. In both the cases, a similar spectral change is observed while cooling. Whereas at high-temperatures the frequency-temperature superposition (FTS) principle holds for the α-process with a stretching parameter significantly below 1 it fails below a certain temperature Tx. Below Tx, in the case of supercooled liquids, in addition to a broadening of the α-relaxation peak the excess wing appears, and the corresponding power-law exponents β (α-peak) and γ (excess wing) of the distribution of correlation times show a similar dependence on the time constant τα, explicitly 1/β and 1/γ both are linear in lgτα. In the PC systems studied, an excess wing is missing and the failure of the FTS principle for the α-relaxation peak directly shows up below Tx. Again, the parameter of the Cole–Davidson susceptibility 1/βCD is linear in lgτα below Tx, and constant above, allowing to identify Tx. In PC phases the crossover temperature Tx may be found much closer to the glass transition temperature Tg as compared to supercooled liquids, and thus can be well studied by standard dielectric spectroscopy.