Thermal conductivity of metal-organic framework 5 (MOF-5): Part I. Molecular dynamics simulations

The phonon thermal conductivity of MOF-5, a metal-organic framework crystal with a phenylene bridge, is predicted between temperatures of 200 K and 400 K using molecular dynamics simulations and the Green–Kubo method. The simulations are performed using interatomic potentials obtained using ab initio calculations and experimental results. The predicted thermal conductivity of MOF-5 is low for a crystal, 0.31 W/m K at a temperature of 300 K, and its temperature dependence is very weak. By decomposing the thermal conductivity into components associated with short- and long-range acoustic phonons, and optical phonons, the weak temperature dependence is found to be related to the mean free path of the majority of phonons, which is of the order of lattice parameter (and is essentially temperature independent). To interpret the results, an analytical thermal conductivity relation is derived, which reduces to the Cahill–Pohl and Slack models under appropriate assumptions. The relation contains a critical frequency, which determines the relative contributions of the short- and long-range acoustic phonons. The small long-range acoustic phonon contribution is found to be related to the long and flexible phenylene bridge, and to the mass mismatch between the cages and the bridges.