Low temperature hcp to monoclinic structural transition in solid C₇₀: ephemeral nature of the intermediate phase

Abstract

We follow the structural transformation in solid C₇₀ from the high temperature hcp to a low temperature monoclinic phase using detailed X-ray diffraction studies at controlled cooling rates from 0.0033 to 0.42 K/min. Rapid cooling of the sample gives the signature of a two-step transformation which disappears when the system is transformed quasi-statically. These experimental results can be rationalized using a simple mean field, Langevin dynamical theory using a free energy functional with minima corresponding to the parent and two competing product phases such that one of these product phases remains metastable throughout. The implication of our results on the existence of the intermediate phase in the sequence, hcp-dhcp- monoclinic, of structural transitions in solid C₇₀ with the lowering of temperature is discussed.