The influence of point defects on the dielectric response of C₆₀ fullerite

C60 samples as well as C60 containing O2, C70 or both were studied. The dielectric loss and constant of the pressed pellets have been measured at 1 and 10 kHz as the temperature was cycled between 85 and 273 K. The low-temperature parts of the dielectric-loss spectra of doped C60 contain Debye relaxational features that have been analysed by other authors. Our data suggest that the electric dipoles responsible relate not only to intrinsic orientational defects but also to dissolved O2. At the higher temperature of 256 K, sharp peaks appeared in the spectra of both dielectric loss and constant, but only during the heating cycle and only if the sample had previously stayed at 85 K for 2 h. Our explanation for their total absence from the cooling curves invokes the masking effect of the random dipolar fields arising from orientational disorder, suppressed by annealing at below 90 K.