Anisotropic Lattice Thermal Conductivity and Suppressed Acoustic Phonons in MOF-74 from First Principles

The thermal transport properties of metalorganic frameworks (MOFs) developed for molecular storage and catalytic separations play an important role in adsorption or catalysis processes but are rarely reported. We calculate the anisotropic thermal conductivities (kappa) of water-stable Zn-MOF-74 with the Boltzmann transport equation and the density-functional-based tight-binding (DFTB) method, which allows for a sufficiently large number of atoms in the simulations without much compromise on accuracy. We find an anisotropic kappa of 0.44 and 0.68 W/m K at 300 K, across and along the pore directions, with acoustic contributions of 8% and 30%, respectively. These unusually low acoustic contributions are explained by the rattling-like behavior of phonons with large vibrational amplitude, low group velocity, and large scattering rate, which are caused by the unique 1-D tubing bundle structure. On the other hand, the cylindrical pores enable larger phonon speed and higher directional structural rigidity along the pore direction, leading to a higher kappa. The frequency-accumulated, directional kappa is explained using the spectral analysis and correlated to the structure characteristics.