AbstractPolymorphism, the ability of a material to exist in multiple structures, is a general phenomenon that is important in a wide range of fields from single crystal organic electronics to pharmaceuticals, food and explosives. This extensive impact makes predicting the existence and stability of polymorphs valuable, yet due to the large configurational space and small energy differences it remains a challenge. Here the two normal pressure polymorphs of coronene, a model molecular system for nanographenes and disk like organic semiconductors, are investigated by Raman spectroscopy and dispersion corrected Density Functional Theory (DFT-D).
Gamma-coronene is the naturally occurring form, whilst the low temperature beta-phase, only recently discovered in 2015, requires the application of a magnetic field during crystal growth to form crystals in standard conditions. This work investigates the crystal growth of gamma-coronene, the vibrations of both polymorphs and their thermodynamic phase stabilities. High-purity crystals of gamma-coronene were successfully grown using a physical vapour transport method. Crystal growth of beta-coronene was attempted, but no crystals of beta-
coronene were formed. Instead the beta-phase was accessed by cooling gamma-coronene crystals below the phase transition temperature of ~150 K. Structures of both polymorphs relaxed using DFT-D3 reproduce experimental lattice parameters to within 1.6% (2.1%) for gamma- (beta-) coronene. Raman active intramolecular and lattice vibrations have been followed across the gamma- to beta-phase transition, with DFT-D3 calculations used to interpret the Raman spectra and, on the basis of polarisation dependence of peak positions and intensities, mode assignment performed. The vibrational density of states and dispersion curves are calculated and Gibbs free energies in the harmonic and quasiharmonic approximations determined. At all temperatures and in both approximations gamma-coronene is predicted to be the most stable polymorph with no phase transition indicated. The error in the calculated Gibbs free energies is estimated to be 10-20 meV.
Recommendations for future work are made. Further investigation is required into the crystal growth of gamma-coronene to produce a reliable and reproducible method. Vibrational frequency calculations including anharmonic effects would be beneficial in solving the few remaining mysteries in the intramolecular Raman spectra. Calculating phase stabilities with higher level theories is recommended, specifically relaxing structures and calculating lattice energies using the hybrid many-body interaction method.
|Date of Award
|26 May 2021
|Simon Crampin (Supervisor) & Enrico Da Como (Supervisor)
- Phase stability
- Density functional theory
- Raman spectroscopy
- Crystal growth