The origin of the double peak at 260 K in calorimetric measurements on solid C60

The fcc-sc phase transition in solid C60 in calorimetric measurements is usually manifested as a single sharp peak centered at a temperature near T ≈ 260 K. In the present study it is shown that small amounts (<1 wt%) of impurities such as C70 can give rise to additional features, including a second peak at 10–20 K below the transition temperature. In contrast, the presence or absence of such double-peak anomaly in high-purity (>99.5%) samples is determined by the kinetics of crystallization. We conclude that impurities as well as crystal imperfections are the primary source of the double-peak anomaly.