The diffraction data were integrated using DAWN and processed, with corrections applied for background, meaningful instrument intensity cutoff and polynomial smoothing using the PDFgetX3 software package using the following parameters: qmin = 1 Å -1 , qmax = 22.5 Å -1 , rpoly = 1.24 Å. Structural models were quantitatively refined against XPDF data using the DiffPy-CMI software package . The resolution peak dampening term Qdamp was constrained to Qdamp = 0.035 Å -1 , this value derived from refinement against a Si standard. The delta1 parameter, corresponding to atomic-motion-derived peak broadening in the "high temperature limit" (with a 1/r dependence) was set to 2 Å following an initial refinement and the isotropic displacement parameters (Uiso) were derived from the reported values for Hf clusters, determined from single-crystal data, and set to 0.18 Å 2 for H, 0.075 Å 2 for C and 0.06 Å 2 for O. Uiso for Hf was obtained from refinement of an ex situ sample of hcp UiO-66(Hf) [Figure ] and subsequently fixed at 0.0069 Å 2 . Refinement of isolated cluster models and the optimised full structure model of hcp UiO-66(Hf) was carried out with refinement parameters set to qmin = 1 Å -1 , qmax = 22.5 Å -1 , rpoly = 1.24 Å and rmin = 1.5 Å. XPDF patterns were simulated using the DiffPy-CMI software package, with the optimised parameters as detailed above. Peak tracking and analysis of processed XPDF and powder X-ray diffraction (PXRD) data was performed using the NumPy and SciPy packages.