Wave-like Phonon Transport in Complex Molecular Crystals

Phonon transport plays a vital role in the vibrational dynamics of energetic materials in shockwave- and thermally-induced initiation as well as in broader applications such as radiative cooling through infrared emissions in delignified and densified wood. Despite extensive research on phonon heat conduction in inorganic crystals such as semiconductors and thermoelectric, phonon transport behavior in complex organic molecule crystals has not received as much attention. Here, we study thermal transport in silicon, Cs2PbI2Cl2, and two organic crystals, cellulose Iβ and α-RDX using the phonon gas model, Cahill-Watson-Pohl model (CWP), Allen-Feldman (AF) model, and the Wigner formulation of thermal transport. We demonstrate that the best estimate of the thermal conductivities in Cs2PbI2Cl2, cellulose Iβ, and α-RDX comes from the Wigner formulation, which accounts for both particle-like and wave-like thermal carriers. In Cs2PbI2Cl2, cellulose Iβ, and α-RDX, the wave-like carriers make up 40, 80, and 75% of the overall thermal conductivity, respectively. Moreover, we find that coupled phonon pairs with minor frequency differences have a disproportionately large role in wave-like heat conduction due to the large number of phonon modes with linewidths close to the mean interband spacing. The low-frequency modes involving the vibrations of molecules and rings in cellulose Iβ, and α-RDX are crucial in the off-diagonal thermal conductivity.