The heat transfer properties of the organic molecular crystal -RDX were studied using three phonon-based thermal conductivity models. It was found that the widely used Peierls-Boltzmann model for thermal transport in crystalline materials breaks down for -RDX. We show this breakdown is due to a large degree of anharmonicity that leads to a dominance of diffusive-like carriers. Despite being developed for disordered systems, the Allen-Feldman theory for thermal conductivity actually gives the best description of thermal transport. This is likely because diffusive carriers contribute to over 95% of the thermal conductivity in -RDX. The dominance of diffusive carriers is larger than previously observed in other fully ordered crystalline systems. These results indicate than van-der Waals bonded organic crystalline solids conduct heat in a manner more akin to amorphous materials than simple atomic crystals.

A long-held belief is that shock energy induces initiation of an energetic material through an energy up-pumping mechanism involving phonon scattering through doorway modes. In this paper, a Fermi's golden rule-based 3-phonon theoretical analysis of energy up-pumping in RDX is presented that considers possible doorway pathways through which energy transfer occurs. On average, modes with frequencies up to 102 cm −1 scatter quickly and transfer over 99% of the vibrational energy to other lowfrequency modes up to 102 cm −1 within 0.16 ps. These low-frequency modes scatter less than 0.5% of the vibrational energy directly to modes with significant nitrogen−nitrogen (NN) activity. The midfrequency modes from 102 to 1331 cm −1 further up-pump the energy to these modes within 5.6 ps. The highestfrequency modes scatter and redistribute a small fraction of the vibrational energy to all other modes, which last over 2000 ps. The midfrequency modes between 457 and 462 cm −1 and between 831 and 1331 cm −1 are the most critical for vibrational heating of the NN modes and phenomena, leading to initiation in energetics. In contrast, modes stimulated by the shock with frequencies up to 102 cm −1 dominate vibrational cooling of the NN modes.

In this paper, we examine how bond strain and rotation carry heat in the molecular crystal α‐RDX. We calculate anharmonic lifetimes and then rank order the phonons and their distortions to the lattice by their importance as carriers of thermal energy. The motions of the atoms that constitute the strain and rotation are determined using the phonon mode energy and mode shapes under the harmonic approximation. To draw the distinction between propagating and diffusive carriers, we additionally compare the thermal conductivity estimates from three microscale models: phonon gas model, Cahill‐Watson‐Pohl formula and Allen‐Feldman harmonic theory. We found that the modes that contribute substantially to the thermal conductivity are also the dominant contributors to the strain and rotation of all bonds in α ‐RDX, among which N−N and N−O bonds were found to exhibit the largest strains and rotations. We also found that on average, the N−N bonds exhibit the largest deformation, ∼35% more strain and ∼6% more rotation than N−O bonds. Our calculations confirm that large straining of the N−N and N−O bonds results from relatively larger displacement of the nitrogen atom in the nitro group −NO2.

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