The degree of cluster aggregation is also directly affected by the temperature and duration of the synthesis. Both heating acidic aqueous zirconium solutions, for example during a solvothermal synthesis, and ageing the reaction mixture, increase the amount of bridging μ 2 -OH between metal centres, and also encourages their conversion to μ 3 -O and μ 3 -OH groups. This therefore favours the formation of higher-nuclearity Zr or Hf clusters. Building on this work, we recently showed that by tuning the synthesis conditions of UiO family MOFs, we can produce MOFs with distinct and different framework topologies, caused by the formation of different nuclearities of different zirconium or hafnium metal clusters. Increasing the temperature, concentration of water and concentration of formic acid modulator allowed us to selectively produce UiO family hafnium MOFs containing larger Hf12 metal clusters [hcp (hexagonal close-packed) UiO-66(Hf), hcp and hns (hexagonal nanosheets) UiO-67(Hf)] instead of Hf6 clusters, which form the fcu (face-centred cubic) topology [Figure ]. Our previous work showed that, due to the similarities between the PXRD patterns of the fcu and hcp MOFs, it is not always immediately obvious that a MOF structure with a different cluster has formed. Despite advances in understanding, including studies on the effects of different variables (including acid modulator and water) on the crystallisation of UiO-66(Zr) (and cerium analogues ), a full picture of the formation routes of MOFs -in particular the precrystalline stages of cluster growth -is still far off. In our work on UiO family MOFs, in situ measurements to probe crystallisation of the hcp phase suggested that inorganic (i.e. largely free from organic ligands) pre-crystalline species formed at early stages in the synthesis. Although this inorganic, noncrystalline material is likely to be a key intermediate in the formation and crystallisation of UiO family MOFs, we were unable to determine its identity. Moreover, it is unknown when in the reaction the crucial structural differences between UiO MOF phases emerge (i.e., selectivity of the hcp phase over the fcu); it is unclear whether the different clusters form via different routes, or if they pass through common intermediates; finally, the timing of the cluster interlinking and framework growth, relative to the timing of the formation of the different clusters, has been little explored.