Low-temperature hcp-to-monoclinic structural transition in solid C-70: Ephemeral nature of the intermediate phase

We follow the structural transformation in solid C-70 from the high-tempera ture hcp to a low-temperature monoclinic phase using detailed x-ray diffrac tion studies at controlled cooling rates from 0.0033 to 0.42 K/min. Rapid c ooling of the sample gives the signature of a two-step transformation that disappears when the system is transformed quasistatically. These experiment al results can be rationalized using a simple mean field, Langevin dynamica l theory using a free-energy functional with minima corresponding to the pa rent, and two competing product phases such that one of these product phase s remains metastable throughout. The implication of our results on the exis tence of the intermediate phase in the sequence hcp-dhcp-monoclinic, of str uctural transitions in solid C-70 With the lowering of temperature is discu ssed. [S0163-1829(98)02346-7].