PHASE-TRANSITION IN C-60 OBSERVED BY POSITRON-ANNIHILATION

The measurements of the positron lifetime and of the Doppler broadenin g of the annihilation line have been performed in the fullerene C60 as a function of temperature between 120 and 300 K. Lifetime components tau1 = (360.6 +/- 0.6 ps, 97.8%) and (1.747 +/- 0.010 ns, 2.2%) have b een observed in a pressed C60 sample. Temperature relation for lifetim e tau = 337.2(1 + alphaT) ps was observed with average slope alpha = 2 .3 x 10(-4) K-1. However, a rapid change of tau is observed between 24 0 and 250 K. The S parameter calculated from the Doppler-broadened ann ihilation line is a constant with some indication of change at approxi mately 245 K. These results indicate that the expanding lattice from t he C60 molecules is undergoing a phase transition and the phases coexi st over an approximately 10 K range, as has been observed by x-ray- an d neutron-diffraction experiments. No hysteresis in tau or S was obser ved.