The concept of secondary building units (SBUs) is central to all science on metal‐organic frameworks (MOFs), and they are widely used to design new MOF materials. However, the presence of SBUs during MOF formation remains controversial, and the formation mechanism of MOFs remains unclear, due to limited information about the evolution of prenucleation cluster structures. Here in situ pair distribution function (PDF) analysis was used to probe UiO‐66 formation under solvothermal conditions. The expected SBU—a hexanuclear zirconium cluster—is present in the metal salt precursor solution. Addition of organic ligands results in a disordered structure with correlations up to 23 Å, resembling crystalline UiO‐66. Heating leads to fast cluster aggregation, and further growth and ordering results in the crystalline product. Thus, SBUs are present already at room temperature and act as building blocks for MOF formation. The proposed formation steps provide insight for further development of MOF synthesis.

Hydrothermal synthesis is a well-established method to produce complex oxides, and is a potential interesting approach to synthesize stoichiometric lead-free piezoelectric K 0.5 Na 0.5 NbO 3. Due to challenges in obtaining the desired stoichiometry of this material, more knowledge is needed on how the end members, KNbO 3 and NaNbO 3 , are nucleating and growing. Here we report on the formation mechanisms and growth during hydrothermal synthesis of KNbO 3 and NaNbO 3 by in situ synchrotron powder X-ray diffraction. We show that tetragonal KNbO 3 crystallites form from

Hafnia, HfO2, which is a wide band gap semiconducting oxide, is much less studied than the chemically similar zirconia (ZrO2). Here, we study the formation of hafnia nanocrystals from hafnium...

In situ monitoring of the formation of crystalline phases during conventional hydrothermal synthesis is experimentally challenging. Here, we report an in situ time-resolved synchrotron X-ray diffraction study during hydrothermal synthesis of NaNbO 3 using a high-pressure custom-made capillary cell penetrable to X-rays. The high time resolution (0.1 s) revealed a sequence of transient intermediate phases, including several unknown phases, before the final perovskite NaNbO 3 was formed. These new findings highlight the complexity of the hydrothermal synthesis of NaNbO 3 and demonstrate the potential for obtaining in-depth knowledge of the reactions taking place by time-resolved in situ X-ray diffraction.

The chemistry of ZnAl2O4 nanocrystal nucleation and growth is examined by X-ray scattering methods, and the results challenge the conventional understanding of its preparation by hydrothermal methods. The common assumption that a specific metal to hydroxide ion (M/OH) ratio is necessary to achieve a phase-pure product is shown to be inadequate. Pair distribution function analysis is used to identify distinct precursor structures, providing an understanding of why particular impurity phases are observed under certain M/OH ratios as heating is applied. In situ X-ray diffraction studies then probe the ZnAl2O4 growth in real time, from which optimal synthesis conditions and the influence of impurities is established. It is found that the heating rate plays a dominant role in impurity formation and dissolution. This observation is explored in three different hydrothermal synthesis methods (microwave, autoclave, and supercritical flow) having different intrinsic heating rates, and methodologies to prepare phase-pure ZnAl2O4 were successfully developed in each case. Ultimately, the atomic scale X-ray scattering information provides concrete guidance to tune the crystallite size, band gap, morphology, and defects of ZnAl2O4 nanocrystals in hydrothermal synthesis establishing a bottom up nonempirical approach to synthesis design.