In Situ X‐ray Diffraction Investigation of the Crystallisation of Perfluorinated Ce^IV‐Based Metal–Organic Frameworks with UiO‐66 and MIL‐140 Architectures**

We report on the results of an in situ synchrotron powder X-ray diffraction study crystallisation aqueous medium two recently discovered perfluorinated CeIV -based metal-organic frameworks (MOFs), analogues already well investigated ZrIV UiO-66 and MIL-140A, namely, F4_UiO-66(Ce) F4_MIL-140A(Ce). The MOFs were originally obtained pure form similar conditions, using ammonium cerium nitrate tetrafluoroterephthalic acid as reagents, small variations reaction parameters found to yield mixed phases. Here, we investigate these compounds, varying such temperature, amount protonation modulator nitric coordination acetic acid. When only HNO3 is present environment, F4_MIL-140A(Ce) obtained. Heating preferentially accelerates nucleation, which becomes rate determining below 57 °C. Upon addition AcOH system, alongside , mixed-phased products are always formed faster, no interconversion between phases occurs. In case F4_UiO-66(Ce), crystal growth rate-determining step. A higher favours formation F4_MIL-140A(Ce), whereas increasing F4_UiO-66(Ce). Based results, a new optimised route achieving pure, high-quality phase mild conditions (60 °C, 1 h) also identified.