INFLUENCE OF OXYGEN IMPURITIES ON ELECTRICAL-PROPERTIES OF FULLERENE C-60

We present high temperature dc, ac and contactless microwave conductiv ity results on solid-state C-60 (films and crystals) from room tempera ture up to 850 K. Heating pristine samples, which were exposed to the ambient atmosphere, under dynamic vacuum at first leads to a reduction of the electrical resistance and finally, above approximate to 700 K, to an increase in the resistance. The decrease is ascribed to oxygen desorption and the increase to the chemical reactivity of residual che misorbed oxygen with the C-60 host molecules, respectively. Samples, a nnealed above 800 K, display a reversible temperature dependence of th e resistance. The high temperature regime of their resistance exhibits an activated behaviour with an universal activation energy of 2E(a) = 1.85 +/- 0.04 eV for crystals and films, which is identical to the HO MO-LUMO splitting of the C-60-molecules.