Dielectric permittivity and AC conductivity in polycrystalline and amorphous C₆₀

The dielectric permittivity and AC conductivity of polycrystalline and amorphous C₆₀ 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 O₂ molecular oxygen intercalation because prolonged exposure to ambient air. The conductivity σ of these samples around 300 K depends on the measuring frequency ν as σ ∼ νⁿ with n ≈ 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 C₆₀ samples prepared by sublimation do not contain interstitial O₂, 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.