Existence of an Orientational Electric Dipolar Response in C 60 Single Crystals

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Abstract
The dielectric constant ∈ and conductivity σ of undoped C60 single crystals have been measured as a function of temperature, 10 K < T < 330 K, and frequency, 0.2 kilohertz < f < 100 kilohertz. On cooling below the first-order structural phase transition at 260 K, a Debye-like relaxational contribution to the dielectric response is observed, which requires the presence of permanent electric dipoles. The relaxation rate is thermally activated with a broad distribution of energies centered at 270 millielectron volts. The existence of a dipole moment in C60 is unexpected, because it is precluded by symmetry for the pure ordered cubic phase. These data suggest that the high degree of frozen-in orientational disorder of the C60 molecules is responsible for the existence of electric dipolar activity.