Correlated disorder and defects are ubiquitous in functional materials, playing a crucial role in fast-ion conductors, ferroelectrics and even superconductors. The possibility that defects might arise in the chemically versatile family of metal-organic frameworks (MOFs) has become appreciated only very recently. But if we are ever to exploit these defects to develop MOFs with new functional responses, we need to be able to control and understand the interactions between defects in MOFs.
UiO-66(Hf), one of the canonical MOFs, was discovered last year to contain a large number of missing ligand defects, especially when synthesised using additional acid. We found that additional broad peaks, which broke the symmetry of the ideal, non-defective UiO-66, appeared in the diffraction pattern at very high defect concentrations. The width of these peaks implied that the defect structure in this material is not long-range ordered (i.e. is disordered), and their presence at specific symmetry-forbidden positions means they must result from a correlation of lower symmetry. Our structural models showed that unlike in previous examples of defective MOFs, missing-linker defects alone could not explain the extra features in the diffraction data. However, by also incorporating correlated hafnium cluster absences, we were able to account for the observed diffraction data.
To confirm this model of correlated hafnium cluster absences, we used transmission electron microscopy (TEM), X-ray total scattering, quantum mechanical calculations, and crucially, low-angle anomalous Hf K-edge scattering measurements carried out at I15. These technically-challenging experiments, carried out at a combination of high energy (65.5 keV) and low Q (0.3 Å-1), provided convincing evidence that scattering from hafnium contributes to the symmetry-breaking diffuse scattering, demonstrating UiO-66 does contain correlated hafnium cluster absences.
This is the first reported example of correlated defects in a metal-organic framework, and the presence of correlated defects in such a canonical material suggests that they may be a common feature in the chemistry of MOFs. These correlated defects also have important implications for the properties of MOFs, from gas sorption and transport to their mechanical flexibility.