Perhaps surprisingly, in this work we observed that that reactions at both 120 and 150°C produced the hcp UiO-66(Hf) framework. This contrasts with our previous work, in which we observed that hcp UiO-67(Hf) required a temperature of 150°C to form (and not 120°C), and also with literature, in which fcu UiO-67(Hf) is usually synthesised at lower temperatures including 120°C. As we have shown that, at both temperatures, the double cluster forms, the clue to this behaviour could lie in the framework formation. At both of these temperatures, the growth rates of the intra-cluster peaks do not appear to change with the presence of BDC linker in the reaction. While there is a different concentration of Hf in the linker-containing reaction mixtures, vs the molecular-cluster-only reactions, meaning they cannot be directly compared, this nevertheless suggests that the linkers do not play a critical part in initial cluster formation and become involved later in the reaction through post-cluster-formation ligand exchange (in agreement with previous research on ligand exchange ).