AC Conductivity and Dielectric Response of Polycrystalline and Amorphous C70

Abstract

The AC conductivity and dielectric constant of polycrystalline and amorphous C70 samples were measured in the 75–300 K temperature range and in the 100 Hz to 1 MHz frequency range. For polycrystalline samples, we observe effects caused by O2 intercalation due to prolonged exposure to ambient air. The conductivity σ of these samples around 300 K depends on the measuring frequency ν as a σ ∼ ν n with n ≈ 0.88, implying a strong reduction of DC conductivity to less than 10−12 S/cm. The dielectric constant of polycrystalline samples shows an anomaly at 285 K which is interpreted as due to the transition from its intermediate rhombohedral phase into its monoclinic low‐temperature phase. In contrast with the polycrystalline samples, the amorphous C70 samples prepared by sublimation do not contain interstitial 02, their 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 disappears in the amorphous samples.