Predicting the structures and associated phase transition mechanisms in disordered crystals via a combination of experimental and theoretical methods.

Michael T Ruggiero,J Axel Zeitler,Qi Li,Johanna Kölbel

doi:10.1039/c8fd00042e

Abstract

Disordered materials make up a large proportion 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 systems retain 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 meaningful crystalline structures, 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 in 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.

Highlights

Disordered solids are an important class of materials that have found use in a wide range of elds, from the semiconductor[1] to pharmaceutical industries.[2]
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
3.1 Terahertz time-domain spectroscopy In order to determine the vibrational response of PCB over a wide temperature range, THz-TDS spectra were acquired from 100–350 K and the results are shown

Summary

Introduction

Disordered (amorphous) solids are an important class of materials that have found use in a wide range of elds, from the semiconductor[1] to pharmaceutical industries.[2]. Paper which leads to increased molecular dynamics compared to their crystalline counterparts.[5] the actual structures and associated motions of the molecules within disordered solids remain a topic of debate, as traditional techniques, such as X-ray crystallography, provide little information that can be directly interpreted without signi cant approximations.[6]. Low-frequency (terahertz) vibrational spectroscopy is a powerful tool for investigating solids, as the terahertz radiation is able to excite large-amplitude collective motions of entire molecules within the solid.[8] This leads to both the internal conformational and intermolecular potential energy surfaces being probed simultaneously, providing a valuable measure of the bulk structure and dynamics. Terahertz time-domain spectroscopy (THz-TDS) is the far-IR extension of the more traditional FTIR technique, but unlike FTIR, which probes internal vibrations, there are no standard functional group speci c terahertz vibrations, meaning each solid has a unique spectral ngerprint at terahertz frequencies.[9,10] the interpretation of THzTDS spectra requires signi cant computational modelling, and (at a minimum) the structures of the associated solids are necessary for initialising the simulations.[11,12]

Methods

Results

Conclusion