G. Ghosh | Searcharxiv

arXiv1998

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

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.

cond-mat.mtrl-sci↗

arXiv1998

Low-temperature structural model of hcp solid C$_{70}$

We report intermolecular potential-energy calculations for solid C_${70}$ and determine the optimum static orientations of the molecules at low temperature; we find them to be consistent with the monoclinic structural model proposed by us in an earlier report [Solid State Commun. {\bf 105), 247 (1998)]. This model indicates that the C_5 axis of the molecule is tilted by an angle $\approx$18^o from the monoclinic b axis in contrast with the molecular orientation proposed by Verheijen {\it et al.} [J. Chem. Phys. {\bf 166}, 287 (1992)] where the C_5 axis is parallel to the monoclinic b axis. In this calculation we have incorporated the effective bond charge Coulomb potential together with the Lennard-Jones potential between the molecule at the origin of the monoclinic unit cell and its six nearest neighbours, three above and three below. The minimum energy configuration for the molecular orientations turns out to be at $θ$=18^o, $ϕ$=8^o, and $ψ$=5^o, where $θ$, $ϕ$, and $ψ$ define the molecular orientations.

cond-mat↗