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^-6 Hz-10¹² Hz) 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 T_x. Below T_x, 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 τ_α, explicitly 1/β and 1/γ both are linear in lg τ_α. 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 T_x. Again, the parameter of the Cole-Davidson susceptibility 1/β_CD is linear in lg τ_α below T_x, and constant above, allowing to identify T_x. In PC phases the crossover temperature T_x may be found much closer to the glass transition temperature T_g as compared to supercooled liquids, and thus can be well studied by standard dielectric spectroscopy.