Relaxation pathways for vibrational modes in molecular crystal α-RDX

Understanding of phonon properties in molecular crystals is critical due to their influence over, for instance, the sensitivity of explosives. In particular, phonons are believed to play a key role in the transfer of energy between the vibrational modes which may result in phenomena leading to initiation. In this work, we investigate the mechanisms for relaxation of the modes in the Van der Waals bonded molecular crystal α-RDX, using the ALD formalism for three phonon scattering. We examine pathways for the transfer of energy from the low frequency lattice vibrations to the high frequency intramolecular vibrational modes which result in large bond distortions and concentration of energy in some modes which may lead to initiation. We investigate the contribution of all phonon modes to bond distortions and identify key inter and intra-molecular interactions responsible for the transfer of energy and relaxation of the modes. Our preliminary results indicate that ALD based technique slightly overpredicts the thermal conductivity since only up to third order phonon scattering processes are considered. We also observe that the low frequency modes (< 4 THz) contribute ~90% to the total bond strain, among which N-N and N-O bonds were found to exhibit the largest strains and rotations.