Direct Observation of the Phase Transition in C60 Fullerene

In the third study, the order-disorder phase transition of single-crystal C60 is studied using time-resolved electron microscopy. Solid C60 undergoes a first-order phase transition upon heating to 260 K. This is a structural transition from the low-temperature simple cubic phase to the high-temperature face-centered-cubic phase. In this study, the C60 sample is heated with a femtosecond laser, and the evolution of the structure is tracked using selected-area diffraction. Two distinct components are revealed in the evolution of the crystal structure. The first is associated with a time constant of 1.6 ns and is ascribed to the energy redistribution between different phonon modes. The second, slower component arises from the phase transition. Its timescale is found to decrease with temperature, from 65 ns at about 290 K to 1.5 ns at about 600 K.