PHASE-TRANSITIONS IN SOLID C70 - SUPERCOOLING, METASTABLE PHASES, AND IMPURITY EFFECT

Modulated differential scanning calorimetry of sublimed C70 shows clear evidence for four first-order transitions, two strong and two weak, which we associate with the two different molecular rotational degrees of freedom in majority (equilibrium) and minority (metastable) phases. The latter correspond to two different stacking sequences of close-packed layers, and their relative contributions to the thermal data correlate well with high-temperature x-ray powder-diffraction results. The upper (long axis tumbling) transition in the majority phase exhibits 50 K supercooling when scanned at 2 K/min. C60 impurities at the few % level depress the transition temperatures substantially.