(Invited) Understanding Phonons and Thermal Transport in Metal-Organic Frameworks

Abstract

The thermal conductivity is an important property of metal-organic frameworks (MOFs). For gas storage applications, a too low thermal conductivity might turn out to be a game-stopper. Conversely, for thermoelectric devices the thermal conductivity has to be low enough. The chemical and structural flexibility of MOFs provides handles for conveniently tuning thermal properties. Still, for realizing the full potential of these materials, a detailed understanding of the corresponding structure-to-property relationships is required. For achieving such an understanding, we employ quantum-mechanical and force-field based simulations to study heat transport processes in MOFs both in real as well as in reciprocal space. The former approach relies on analyzing the effective local temperature in different parts of the MOF during a non-equilibrium molecular dynamics simulation. This allows identifying specific elements and bonds within the MOF that act as heat-transport bottlenecks. For a variety of MOF structures we show that the main local inhibitor to heat flow is the bond between the metal ions in the nodes and the carboxylic oxygens in the linkers. Further computer experiments then allow assessing the impact of the strength of that bond and of parameters like the mass of the nodes, the length and density of the linkers, bonding motives, the network topology, etc.. Typically, we find that stronger bonds, lighter nodes, and longer and more dense linkers increase thermal transport. As an alternative approach, analyzing heat transport in reciprocal space enables us to connect the values of the thermal conductivity to the properties of phonons as the quanta of mechanical and thermal lattice energy. Of particular interest in this context is the impact of structural variations on quantities like phonon densities of states, group velocities, and phonon lifetimes, which all directly impact heat transport processes. An analysis of these quantities provides insight into the nature of the vibrations that most strongly impact thermal conductivities. Consistent with the results from the real-space analysis, we find that the group velocities of the relevant phonons are increased by lighter nodes and by an increased mode-linker coupling. Notably, the bonds between nodes and linkers are also a major source of anharmonicities, thus reducing phonon lifetimes and decreasing thermal conductivities. Finally, we show that phonon properties are also crucial for other parameters, like elastic constants, thermal expansion, and - by virtue of electron-phonon coupling - also charge transport.