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.