Low temperature hcp to monoclinic structural transition in solid C$_{70}$ : Is there an intermediate phase?"

TL;DR

This study investigates the structural phase transition in solid C$_{70}$ from hcp to monoclinic, revealing a two-step transformation influenced by cooling rate and discussing the possible existence of an intermediate phase.

Contribution

It provides experimental evidence and a theoretical framework for understanding the temperature-dependent structural transitions in solid C$_{70}$, including the role of metastability and intermediate phases.

Findings

Rapid cooling induces a two-step transformation.

Quasi-static cooling suppresses the intermediate phase.

Theoretical model explains the metastable phases.

Abstract

We follow the structural transformation in solid C$_{70}$ from the high temperature hcp to a low temperature monoclinic phase using 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 transformaed quasi-statically. These experimental results can be rationalised 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 C$_{70}$ with the lowering of temperature is discussed.