A new hexagonal phase of fullerene C60

A new phase of fullerene C60 with a simple hexagonal unit cell with 6/mmm symmetry was grown by slowly evaporating solutions of C60 in dichloromethane. X-ray measurements reveal that the c/a ratio is 1.616 at 298 K and increases as temperature decreases. At low temperature, it seems to extrapolate to 1.633, the ideal ratio for a close-packed hexagonal lattice. The unit cell volume which is higher than that of the usual cubic C60 phases at high temperature decreases near 90 K. A structural model is proposed according to which a 3-fold molecular axis is parallel to the 6-fold crystallographic z-axis. This suggests that the molecules could reorient around this axis at high temperature.