Dielectric permittivity and AC conductivity in polycrystalline and amorphous $\pmb{C_{60}}$

Abstract

The dielectric permittivity and AC conductivity of polycrystalline and amorphous $C_{60}$ samples were measured at temperatures between 75 and 300 K and frequencies in the range 100 Hz to 1 MHz. For polycrystalline samples we observe effects caused by $O_{2}$ molecular oxygen intercalation because prolonged exposure to ambient air. The conductivity $\sigma$ of these samples around 300 K depends on the measuring frequency $\nu$ as $\sigma$ $\sim$ $\nu^{n}$ with n $\approx$ 1 implying a strong reduction of DC conductivity to less than $10^{-12}$ S/cm. Dielectric permittivity in polycrystalline samples shows an anomaly around 258 K due to its order-disorder phase transition and dielectric relaxation phenomena is observed in the range 130-200 K with an activation energy of 0.237 eV. In contrast with the polycrystalline samples, the amorphous $C_{60}$ samples prepared by sublimation do not contain interstitial $O_{2}$, their DC conductivity at 300 K is of about $10^{-6}$ S/cm, is independent of frequency, and is well described by the hopping mechanism (Davis-Mott $T^{1/4}$ law) in the 200-300 K range. All evidence of phase transitions and/or dielectric relaxation disappears in the amorphous samples.