Predicting the Structures and Associated Phase Transitions Mechanisms in Disordered Crystals via a Combination of Experimental and Theoretical Methods

 

“Predicting the Structures and Associated Phase Transitions Mechanisms in Disordered Crystals via a Combination of Experimental and Theoretical Methods.” Faraday Discussions (2018).

 

 

Disordered materials make up a large portion of condensed phase systems, but the difficulties in describing their structures and molecular dynamics limit their potential applications. Disordered crystalline systems, also known as plastic crystals, offer a unique perspective into these factors because the system retains a degree of crystallinity, reducing the degrees of freedom that must be explored when interpreting the results. However, while disordered crystals do diffract X-rays, it is difficult to fully resolve a meaningful crystalline structure, with the best scenario resulting in lattice parameters. In this study, we use a combination of experimental terahertz time-domain spectroscopy, and theoretical solid-state ab initio density functional theory and molecular dynamics simulations to fully elucidate the structures and associated dynamics of organic molecular solids. The results highlight that this combination provides a complete description of the energetic and mechanistic pathways involved in the formation of disordered crystals, and highlights the importance of low-frequency dynamics on their properties. Finally, with structures fully determined and validated by the experimental results, recent progress into anharmonic calculations, namely the quasi-harmonic approximation method, enables full temperature and pressure-dependent properties to be understood within the framework of the potential energy hyper-surface structure.

 

 

“A corresponding pellet containing pure polyethylene was also made to act as a standard blank for absorption measurements. All experimental THz-TDS spectra were obtained using a commercial TeraPulse 4000 spectrometer. Variable-temperature measurements were performed using a liquid nitrogen cryostat (Janis, Massachusetts, USA) equipped with a externally controlled heating element (Lakeshore 330, Ohio, USA).”