A significant misfit in the region around 4.5 Å was found in this initial refinement [Figure (a)] that was not well fit by the isolated tetramer cluster model. This initial model did not take account of the likely structuring of water as a coordination sphere around the cluster. We therefore augmented our tetramer model by including an additional coordination shell of 24 water molecules, with the oxygens placed using the locations of water and chloride in the crystal structure of zirconyl chloride octahydrate [Figure ]. The simulated XPDF of this augmented tetramer The XPDF analysis of our DFW 65:25:10 solvent dataset corroborates that a change of solvent system, without any change in temperature, is already sufficient to cause some degree of cluster aggregation, from the tetramer to the fcu UiO-type single cluster. However, the double cluster was not observed in any of these studies. The single cluster species is known to be particularly stable, as well as being found in a wide range of syntheses and conditions, which suggests that changing the type and concentration of modulating acid, or the reaction time, are not sufficient alone for further cluster aggregation; higher temperatures may also be required to encourage the formation of higher-nuclearity clusters. Behaviour of Hf in DMF : formic acid : water solvent at raised temperatures