The two-stage framework growth of hcp UiO-66(Hf) at 120°C described in this work may, however, shed light on our previous discovery that an ex situ reaction with a UiO-67(Hf) reaction mixture at 120°C resulted in a poorly crystalline material, rather than either the fcu or the hcp framework. If that previous ex situ UiO-67(Hf) reaction mixture follows a similar framework growth pattern to the two-stage pattern shown in this work, then the first stage of framework growth could occur before the cluster aggregation from single to double clusters is complete. This could involve the cluster coordination sites being 'blocked' against further attack by exchange of dicarboxylic acid linkers with the cluster-terminating ligands, resulting in a mixture of coordinated single and double clusters, with different and less compatible coordination geometries.