The relaxation dynamics of water in hygroscopic molecular solids is probed by broadband dielectric spectroscopy in the temperature range from 200 to 450 K. Evidence is found for three types of dynamic processes. The intermediate process is common to all probed materials and is associated with the reorientation of bound water molecules that are attached directly onto organic molecules and counterions. A faster process is observed in rhodamine chloride and fullerol, which is the dynamic signature of water in higher hydration layers, either at grain boundaries (rhodamine) or in interstitial clusters (fullerol). All these processes are observed near room temperature and exhibit nonmonotonic temperature dependence and decreasing spectral strength upon heating. In fullerol a third, ultraslow relaxation is observed at high temperature, which may be due to the reorientation of water–fullerol complexes or to the diffusion of water vapor through intermolecular voids.

Impedance spectroscopy is employed to probe the frequency-dependent conductivity and dielectric response of the C60O24Na24 fulleride. The conduction properties of this organic material match the so-called universal behavior of the ac conductivity in disordered media [Dyre, J. C.; Schroder, T. B. Rev. Mod. Phys. 2000, 72, 873–892]. In the whole temperature range studied, electrical conduction is due to intermolecular hopping processes of electronic charge carriers, characterized by an effective activation energy ranging between 0.7 eV at room temperature and 0.9 eV at 475 K. A single dielectric loss feature is observed, associated with the hopping of charge carriers surrounded by a polarization cloud. The polarizability of the material is mainly due to the distortion of the ionic O–Na bonds of the fullerene derivative, which are tight enough that no ionic contribution to the conductivity is observed up to the highest temperature probed (550 K).

Impedance spectroscopy is employed to probe the impact of water on the dc conductivity and ac dielectric response of the polycrystalline C 60 (ONa) 24 fulleride, both in its bulk-hydrate form, stable only below 370 K, and in the pure form, obtained by heating to high temperature. Exposure of the pure material to ambient air results in the condensation of water vapor on the crystallites' surface, which in turn leads to an enhancement of the room-temperature conductivity by four orders of magnitude due to charge transport through the hydration layer. Electrical conduction in the hydrate between 320 and 380 K is dominated by a non-equilibrium contribution associated with the structural water, which leads to a value of the dc conductivity that is higher than that of the pure material by almost two decades at 360 K. Both conductivity enhancements are most likely due to a proton exchange mechanism. All impedance spectra exhibit, in the radiofrequency range, a dielectric loss feature related to the accumulation of free charges at grain boundaries, whose strength is strongly affected by the presence of hydration water.

We study the temperature-dependent molecular dynamics, ion conduction, and phase behavior of plastic-crystal electrolytes based on the succinonitrile molecule. We employ calorimetry and dielectric spectroscopy to probe binary mixtures of succinonitrile with glutaronitrile or acetonitrile and also analyze the effect of dissolving lithium salts in these systems. The glutaronitrile-succinonitrile mixture has the highest conductivity, and it is the only plastic-crystal system that displays a perfect correlation between the ion drift and the on-site reorientational dynamics. Doping with lithium ions boosts the conductivity but breaks such perfect correlation. All of these features can be rationalized by assuming that conduction is due to the self-diffusion of a minority of ionized dinitrile molecules. Doping with lithium salts slows down the collective molecular dynamics while leaving the intramolecular relaxation motion unaffected. All samples exhibit a very broad melting transition and exist in a mixed liquid plus plastic state near room temperature. Some mixtures undergo phase segregation below 233 K, the transition temperature between the plastic and the fully ordered solid phase in pure succinonitrile, resulting in the appearance of a space-charge relaxation loss. Phase separation therefore plays an important role in pristine and lithium-doped succinonitrile mixtures.Postprint (published version

We probe the ion conduction and the molecular dynamics in a pure and lithium-salt doped dinitrile molecular plastic crystal. While the diffusion of the Li + ions is decoupled from the molecular reorientational dynamics, in the undoped plastic crystal the temperature dependence of the mobility of dinitrile ions and thus of the conductivity is virtually identical to that of on-site molecular rotations. The undoped material is found to obey the Walden and Stokes-Einstein rules typical of ideal liquid electrolytes, implying that an effective viscosity against diffusion can be defined even for a plastic crystalline phase. These surprising results, never reported before in a translationally ordered solid, indicate that in this dinitrile plastic crystalline material the time scale of translational diffusion is perfectly correlated with that of the purely reorientational on-site dynamics.