As one of the most abundant materials in the world, calcium carbonate, CaCO3, is the main constituent of the skeletons and shells of various marine organisms. It is used in the cement industry and plays a crucial role in the global carbon cycle and formation of sedimentary rocks. For more than a century, only three polymorphs of pure CaCO3—calcite, aragonite, and vaterite—were known to exist at ambient conditions, as well as two hydrated crystal phases, monohydrocalcite (CaCO3·1H2O) and ikaite (CaCO3·6H2O). While investigating the role of magnesium ions in crystallization pathways of amorphous calcium carbonate, we unexpectedly discovered an unknown crystalline phase, hemihydrate CaCO3·½H2O, with monoclinic structure. This discovery may have important implications in biomineralization, geology, and industrial processes based on hydration of CaCO3.

An effective search for new organic crystals for prospective use in nonlinear optical (NLO) applications requires quantitative and fast experimental determination of their NLO properties at a molecular level. However, the growth of sufficiently large single crystals, which are needed for structural analysis and refinement by X-ray methods, is a time-consuming and sometimes impossible task. Single crystals of a considerably smaller size may be effectively used for complete structural analysis by electron diffraction combined with simulation methods. When the crystal structure of a given compound is known, its NLO properties may be estimated using quantum-chemical methods for calculation of the molecular nonlinearity tensor and the relationships between its components and the macroscopic coefficients of the crystalline nonlinearity tensor. In the present work, the semiempirical PM-3 method was employed for this aim.

Within this article, it is shown that an electrochemical defluorination and additional fluorination of Ruddlesden−Poppertype La 2 NiO 3 F 2 is possible within all-solid-state fluoride-ion batteries. Structural changes within the reduced and oxidized phases have been examined by X-ray diffraction studies at different states of charging and discharging. The synthesis of the oxidized phase La 2 NiO 3 F 2+x proved to be successful by structural analysis using both X-ray powder diffraction and automated electron diffraction tomography techniques. The structural reversibility on re-fluorinating and re-defluorinating is also demonstrated. Moreover, the influence of different sequences of consecutive reduction and oxidation steps on the formed phases has been investigated. The observed structural changes have been compared to changes in phases obtained via other topochemical modification approaches such as hydride-based reduction and oxidative fluorination using F 2 gas, highlighting the potential of such electrochemical reactions as alternative synthesis routes. Furthermore, the electrochemical routes represent safe and controllable synthesis approaches for novel phases, which cannot be synthesized via other topochemical methods. Additionally, side reactions, occurring alongside the desired electrochemical reactions, have been addressed and the cycling performance has been studied.

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Concrete is the most prevalent manufactured material that has shaped the built environment, but the high-temperature production of cement, the main component of concrete, has a massive carbon footprint. It is shown that CO 2 emissions during clinker production of cement can be circumvented by a metathesis reaction at room temperature in ball-mills, where the cement clinker is replaced by non-calcined limestone and alkali-activated binders/ geopolymers. An amorphous intermediate (aNaSiCC) containing a random mixture of the ionic constituents in "molecular" dispersion is formed by mechanochemical activation of CaCO 3 and Na 2 SiO 3 . This allows molecular transport during crystallization and low activated reactions, as precipitation of solids from liquids (nucleation limited and kinetically controlled) and solid-state transformations (diffusion-limited and thermodynamically controlled) have equal weight. Several steps of the hydration reaction could be resolved. Activating the amorphous aNaSiCC precursor with NaOH leads to a CSH-like phase with a C/S ratio of ≈1 containing some sodium. The carbonate components pass through a multistep crystallization from aNaSiCC via pirssonite and gaylussite to monohydrocalcite. The findings help unravel the interplay between thermodynamics and kinetics in complex reactions of alkali-activated binders and for CaCO 3 crystallization in industrial and geochemical settings, where dissolved silicate is always involved.