Unraveling the Mechanism for Phonon Based Heat Transport in Metal-Organic Frameworks

Heat transport is a crucial process for many applications of metal-organic frameworks (MOFs), including gas adsorption, catalysis, and bio-encapsulation. A prerequisite for designing MOFs with optimal thermal conductivities is developing a profound understanding of the underlying atomistic mechanisms. This is at the heart of the present publication, where we first show that the common approach of describing heat conduction in crystalline insulators as a particle-like propagation of phonons (as vibrational energy quanta) is insufficient. This is the consequence of strong inter-band couplings between the densely lying phonon bands in the highly complex materials considered here (MOF-5 and HKUST-1). When accounting for that through suitable physical models in combination with highly accurate machine-learned potentials, a quantitative agreement with experimental single-crystal data becomes possible. Interestingly, the phonons dominating particle-like transport are primarily (acoustic) modes at very low frequencies (up to ∼1.5 THz), while inter-band transitions also occur at much higher frequencies. This can be explained by the sharp drop of phonon lifetimes with frequency, which diminishes particle-like transport but promotes inter-band coupling. Our analysis not only rationalizes the differences in thermal conductivities between MOF-5 and HKUST-1, but also lays the foundations for developing a phonon-based understanding of thermal transport processes in MOFs.