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.