Unified thermal transport in Molecular crystals

Phonons play an essential role in heat transport and energy conversion in molecular crystals. Moreover, phonons are vital in understanding the sensitivity and laser-induced properties of energetic materials. However, the phonon transport mechanism in many molecular crystals is still unclear. This work studies the anharmonic phonon properties and thermal transport of molecular crystal RDX and cellulose Iβ using lattice dynamics and molecular dynamics. Both materials have a large number of phonons whose path lengths fall below the Ioffe-Regel limit, where the widely used phonon gas model (PGM) has been shown to break down. The unified thermal transport model, which accounts for the contributions from diagonal and off-diagonal terms of the heat-current operator, is applied to study the thermal conductivity. The unified model shows better agreement with the Green-Kubo method than PGM for both materials. Compared to the diagonal terms (particle-like), the off-diagonal terms (wave-like) dominate the phonon transport in both materials. Our study provides insights into the understanding of thermal transport in complex molecular crystals and paves the way for theoretical study of light matter interactions in complex molecular materials.