Hopping Conductivity and Polarization Effects in a Fullerene Derivative Salt

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).