The conditions for the synthesis of the hcp UiO-66(Hf) MOF were adapted from Ref. 54, with quantities reduced to account for the smaller in situ reaction volume. The concentrations of HfCl4 and, where relevant, organic linker, were increased to improve signal to noise. The compositions of the reaction mixtures are described below. HfCl4 (4.8 mg, 0.015 mmol) and terephthalic acid (H2BDC) (Alfa Aesar, 98 %, 2.5 mg, 0.015 mmol) were added to a capillary, followed by dry N,N-dimethylformamide (Sigma Aldrich, 99.85 % anhydrous DMF) (65 μL), formic acid (Fisher, 98/100 %) (25 μL) and water (10 μL), then sonicated for 10 minutes. We also carried out reactions without the terephthalic acid ligand, to assess its importance in cluster formation. HfCl4 (116 mg, 0.362 mmol) was sonicated for 10 minutes with dry DMF (6.5 mL), formic acid (2.5 mL) and water (1.0 mL). 0.1 mL of the resulting solution was measured into the capillary.