Structural Transitions of the Metal-Oxide Nodes within Metal–Organic Frameworks: On the Local Structures of NU-1000 and UiO-66

In
situ pair distribution function (PDF) analyses and density functional
theory (DFT) computations are used to probe local structural transitions
of M6O8 nodes found in two metal organic frameworks
(MOFs), NU-1000 and UiO-66, for M = Zr, Hf. Such transitions are found
to occur without change to the global framework symmetry at temperatures
within a range relevant to many potential MOF applications. For the
particular M6(O)8 nodes studied here, the observed
distortions can be mapped to polymorphic forms known for bulk ZrO2. In the MOF framework, however, node distortions are found
to occur at substantially lower temperature than analogous distortions
in bulk ZrO2 owing to the nanoscale nature of the former.