Spatially Resolving Anharmonic Lattice Dynamics in Molecular Crystals With X-Ray Diffraction and Terahertz Spectroscopy

The combination of X-ray diffraction and low-frequency vibrational spectroscopy has proven to be a powerful method for understanding the relationship between molecular and crystalline structures, dynamics, and the properties of materials. In this work, we show how information obtained from terahertz time-domain spectroscopy (THz-TDS) measurements, coupled with first-principles simulations including anharmonic effects, is able to reconcile specific vibrational motions to the experimentally observed large- amplitude thermal displacements in a pair of isomeric molecular crystals. In particular, we show that a single terahertz mode is responsible for the observed structural data, and provide a framework for predicting and interpreting the origins of related phenomena.