Study of phase transition in solid C70 at low temperatures

The structural phase transition in high quality ideal h.c.p. solid C-70 at low temperature has been reported. At a slow cooling-rate, typically, 0.005 K/min., the h.c.p. solid transforms in a single step, to monoclinic with onset at 270 K and Delta Tc similar to I00 K. Delta Tc increases for higher cooling-rates and above certain rate an intermediate phase appears, resembling the two-step transition reported in literature. A mean-field calculation indicates that the intermediate phase is metastable. It also emerges from this calculation that if the cooling-rate is extremely slow, Delta Tc disappears and the corresponding low temperature structure has the global minimum potential energy. Detailed potential energy calculations have been carried out to determine the minimum energy configuration of the C-70 molecules in the low temperature monoclinic structure.