Complex bond distortion behaviors of anharmonic thermal carriers in van der Waals bonded molecular crystal α-RDX

Understanding of phonon properties in molecular crystals is critical due to their influence over, for instance, sensitivity of explosives, and charge separation in organic semiconductors. In particular, hot-spot formation in shocked energetic materials, are known to follow from bond scission events. However, the phonon-mediated mechanisms that promote bond breaking are still unclear. In this work, we examine how distortion of bonds carry heat in van der Waals bonded molecular crystal α-RDX. We rank order the phonons and their distortions to the lattice by their importance as carriers of heat. The motions of the atoms that constitute the bond distortions are determined using the phonon mode energy and mode shapes under the harmonic approximation. We also estimate the thermal conductivity of α-RDX using the Allen-Feldman (AF) harmonic theory. The AF model takes into account the contribution of highly anharmonic diffusive carriers which dominate thermal transport in many molecular crystals. Our preliminary results indicate that the the low frequency modes (< 4 THz) contribute ~90% to the total thermal conductivity as well as ~90% to the total bond strain, among which N-N and N-O bonds were found to exhibit the largest strains and rotations.