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Zeolites are microporous crystalline materials that find a very wide range of applications, which, however, are limited by the size and dimensionality of their pores. Stable silica zeolites with a three‐dimensional (3D) system of extra‐large pores (ELP, i.e. pores with minimum windows along the diffusion path consisting of more than 12 SiO4/2 tetrahedra, 12R) are in demand for processing larger molecules than zeolites can currently handle. However, they have challenged worldwide synthetic capabilities for more than eight decades. In this review we first present a brief history of the discovery of ELP zeolites. Next, we show that earlier successes of zeolites with 3D ELP were not actually zeolites, but rather interrupted structures with, in addition, a composition that severely detracted from their stability. Finally, we present three new fully connected stable silica‐based 3D ELP zeolites ZEO‐1, ZEO‐3 and ZEO‐5, discuss their preparation methods and stability as well as the clear advantage of their increased porosity in catalysis and adsorption processes involving large molecules. We will discuss peculiar characteristics of their preparation and present two new reaction types giving rise to zeolites (1D‐to‐3D topotactic condensation and interchain expansion), highlighting how new synthesis methods can provide materials that would otherwise be unfeasible. [ABSTRACT FROM AUTHOR]

An organic cation lacking specificity in its structure-directing action offers the possibility, through the screening of other structure-directing parameters, to synthesize a variety of zeolites. In this work we show that the organic structure-directing agent 2-isopropyl-1,3-dimethylimidazolium (2iPr13DMI) can produce up to seven different zeolite phases depending on water concentration, the presence of inorganic impurities, crystallization temperature and time, and germanium molar fraction. The obtained phases are very different in terms of pore system, connectivity of the zeolite structure and structural units. At the pure SiO2 side, ZSM-12 and SSZ-35 dominate, with ZSM-12 being favored by the presence of potassium impurities and by less concentrated conditions. The introduction of Ge at low levels favors SSZ-35 over ZSM-12 and as the Ge fraction increases it successively affords CSV, -CLO and two distinct UOS zeolites, HPM-11 and HPM-6. These two zeolites have the same topology but distinct chemical compositions and display powder X-ray diffraction patterns that are much different from each other and from that of as-synthesized IM-16 (UOS reference material). They also show different symmetry at 96 K. Rietveld refinements of the three as-made UOS materials mentioned are provided. HPM-6 and HPM-11 are produced in distinct, non-adjacent crystallization fields. The frequent cocrystallization of the chiral STW zeolite, however, did not afford its synthesis as a pure phase. Molecular mechanics simulations of the location of the organic cation and host–guest interactions fail to explain the observed trends, but also considering the intrinsic stability of the zeolites and the effect of germanium help to rationalize the results. The study is completed by DFT calculations of the NMR chemical shifts of 13C in UOS (helping to understand splittings in the spectrum) and 19F in CSV (supporting the location of fluoride inside the new [4452], which is an incomplete double 4-ring). [ABSTRACT FROM AUTHOR]

Lead‐free piezoelectric materials with excellent performance are greatly demanded for the environmental issues. The lead‐free ferroelectric single crystals are therefore greatly concerned for their much higher piezoelectricity than the polycrystalline counterparts and the domain‐tunable property. However, the growth of large size and high‐quality lead‐free ferroelectric crystals has always been a great challenge. In this work, a ferroelectric KTa1‐xNbxO3 (KTN) single crystal with an extremely large size of 25 × 25 × 37 mm3 was grown using the modified top‐seeded solution growth method. The large size enabled us to carry out comprehensive investigations on the crystal. The complete sets of dielectric, piezoelectric, and elastic constants for the [001]C‐poled KTN crystal with quasi single domain state were determined. Results showed that the crystal exhibits high electromechanical coupling coefficients with kt~0.642 and k33~0.679. Furthermore, the orientation dependence of the piezoelectric, dielectric, and elastic properties was determined by the tensor matrix transformation technique, and the maximum d33, s33E, and k33 were found along the polar direction [001]C. This work not only provides important guidance for the growth technology of lead‐free ferroelectric crystals but also offers essential datasets of lead‐free alkali niobate system, which is of great importance for both theoretical studies and designs of electromechanical devices. [ABSTRACT FROM AUTHOR]