Thermodynamic Insight in the High-Pressure Behavior of UiO-66: Effect of Linker Defects and Linker Expansion

In this Article, we present a molecular-level
understanding of
the experimentally observed loss of crystallinity in UiO-66-type metal–organic
frameworks, including the pristine UiO-66 to -68 as well as defect-containing
UiO-66 materials, under the influence of external pressure. This goal
is achieved by constructing pressure-versus-volume profiles at finite
temperatures using a thermodynamic approach relying on ab initio derived
force fields. On the atomic level, the phenomenon is reflected in
a sudden drop in the number of symmetry operators for the crystallographic
unit cell because of the disordered displacement of the organic linkers
with respect to the inorganic bricks. For the defect-containing samples,
a reduced mechanical stability is observed, however, critically depending
on the distribution of these defects throughout the material, hence
demonstrating the importance of judiciously characterizing defects
in these materials.