STRUCTURAL PHASE-TRANSITIONS IN HCP C-70 SINGLE-CRYSTALS

The thermal expansion of vapor-grown C-70 single crystals has been inv estigated using high-resolution capacitance dilatometry from 5-380 K. Measurements were made both parallel and perpendicular to the hexagona l c-axis. Three first-order phase transitions which we associate with the consecutive disordering of the C70 molecules are observed upon hea ting at almost-equal-to 280 K (long-axis spinning), almost-equal-to 30 0 K (long-axis precession) and almost-equal-to 355 K (quasi-free rotat ion), respectively. The highest-temperature transition exhibits a very large (almost-equal-to 50 K) thermal hysteresis. Powder and single-cr ystal X-ray diffraction show that the crystals are predominantly hexag onal-close-packed HCP) with an ideal c/a almost-equal-to 1.63 above (3 60 K and c/a almost-equal-to 1.84 at 295 K.