Phonon-Focusing and Rattler-Mode Interference in Thermal Conductivity Transitions of the Breathing Metal-Organic...

The metal-organic framework (MOF) MIL-53(Al) has a framework geometry with an unconstrained wine-rack mode that enables it to concertina between large-pore (lp) and narrow-pore (np) structures either under pressure or with the uptake of adsorbates. This article presents the results of equilibrium classical molecular dynamics simulations that show that, in these breathing MOFs, the thermal conductivity─an important property for gas sorption in high-surface-area materials─also changes between open and closed conformations. The observed conductivity change differs significantly from that of a network of resistors undergoing the same geometric change. The dispersion relations and phonon group velocities in both the lp and np configurations of MIL-53(Al) are computed using self-consistent-charge density functional-based tight binding (SCC-DFTB). These exhibit rattler-mode behavior and phonon-focusing effects. To provide a mechanistic understanding of these features, a reduced order model of the MOF architecture is presented that captures the lattice dynamics and the band avoidance of acoustic and rattler phonon bands. Moreover, it is shown that the observed phonon focusing is caused by band bending from the rattler modes.