Phase transition in ${\mathrm{C}}_{60}$ observed by positron annihilation

The measurements of the positron lifetime and of the Doppler broadening of the annihilation line have been performed in the fullerene ${\mathrm{C}}_{60}$ as a function of temperature between 120 and 300 K. Lifetime components ${\mathrm{\tau }}_1$=(360.6±0.6 ps, 97.8%) and (1.747±0.010 ns, 2.2%) have been observed in a pressed ${\mathrm{C}}_{60}$ sample. A temperature relation for lifetime τ=337.2(1+αT) ps was observed with average slope α=2.3×${10}^{\mathrm{-}4}$ ${\mathrm{K}}^{\mathrm{-}1}$. However, a rapid change of τ is observed between 240 and 250 K. The S parameter calculated from the Doppler-broadened annihilation line is a constant with some indication of change at ∼245 K. These results indicate that the expanding lattice from the ${\mathrm{C}}_{60}$ molecules is undergoing a phase transition and the phases coexist over an ∼10 K range, as has been observed by x-ray- and neutron-diffraction experiments. No hysteresis in τ or S was observed.