Effect of Graphene on Ice Polymorph

Recently, ice with the stacking disorder structure, consisting of random sequences of cubic ice (Ic) and hexagonal ice (Ih) layers, is reported to be more stable than pure Ih/Ic. While, due to a much lower free energy barrier of heterogeneous nucleation, in practice, the freezing process of water is usually controlled by heterogeneous nucleation which is triggered by an external medium. Herein, molecular dynamic simulations were carried out to explore the polymorph dependence of ice on the lattice structure of substrates. It turns out that, during the nucleation stage, the polymorph of ice nuclei can be severely altered by the graphene substrate, on which the Ih was found to occupy an absolute majority in new-formed ice. This can be attributed to the structure similarity between graphene and basal face of Ih. Besides the nucleation stage, our results suggest that the substrate can not affect the polymorph of ice which is far from the graphene surface. The polymorph selectivity of graphene to Ih will diminish with the growth of ice layer.

Visualization of clay minerals at the atomic scale

This review demonstrates that high-resolution transmission electron microscopy (HRTEM) imaging of clay minerals or phyllosilicates with an incident electron beam along the major zone axes parallel to the constituting layers, in which the contrast corresponds to individual cation columns in the images obtained, is indispensable for elucidating the enigmatic structures of these minerals. Several kinds of variables for layer stacking, including polytypes, stacking disorder and the interstratification of various kinds of unit layers or interlayer materials, are common in phyllosilicates. Local and rigorous determination of such variables is possible only with HRTEM, although examination as to whether the results obtained by the HRTEM images from limited areas represent the whole specimen should be made using other techniques, such as X-ray diffraction. Analysis of these stacking features in clay minerals provides valuable insights into their origin and/or formation processes. Recent state-of-the-art techniques in electron microscopy, including incoherent imaging, superior resolutions of ~0.1 nm and low-dose imaging using new recording media, will also contribute significantly to our understanding of the true structures of clay minerals.

New zeolite-like RUB-5 and its related hydrous layer silicate RUB-6 structurally characterized by electron microscopy

This study made use of a recently developed combination of advanced methods to reveal the atomic structure of a disordered nanocrystalline zeolite using exit wave reconstruction, automated diffraction tomography, disorder modelling and diffraction pattern simulation. By applying these methods, it was possible to determine the so far unknown structures of the hydrous layer silicate RUB-6 and the related zeolite-like material RUB-5. The structures of RUB-5 and RUB-6 contain the same dense layer-like building units (LLBUs). In the case of RUB-5, these building units are interconnected via additional SiO4/2 tetrahedra, giving rise to a framework structure with a 2D pore system consisting of intersecting 8-ring channels. In contrast, RUB-6 contains these LLBUs as separate silicate layers terminated by silanol/siloxy groups. Both RUB-6 and RUB-5 show stacking disorder with intergrowths of different polymorphs. The unique structure of RUB-6, together with the possibility for an interlayer expansion reaction to form RUB-5, make it a promising candidate for interlayer expansion with various metal sources to include catalytically active reaction centres.

HPM-14, a New Germanosilicate Zeolite with Interconnected Extra-Large Pores Plus Odd-Membered and Small Pores

We describe HPM-14, a new extra-large pore zeolite synthesized using imidazolium-based organic structure-directing agents, fluoride anions and germanium and silicon as tetrahedral components of the framework. Due to the presence of stacking disorder, the structure elucidation of HPM-14 was challenging, and different techniques were necessary to clarify the details of the structure and to understand the nature of the disorder. The structure has been solved by three-dimensional electron diffraction technique (3D ED) and consists of an intergrowth of two polymorphs possessing a three-dimensional channel system, including an extra-large pore opened through windows made up of sixteen tetrahedral atoms (16-membered ring, 16MR) as well as two additional sets of odd-membered (9MR) and small (8MR) pores. The intergrowth has been studied by scanning transmission electron microscopy (C s -STEM) and powder X-ray diffraction simulations (DIFFaX), which show a large predominance of the monoclinic polymorph A.<br>

HPM-14, a New Germanosilicate Zeolite with Interconnected Extra-Large Pores Plus Odd-Membered and Small Pores

We describe HPM-14, a new extra-large pore zeolite synthesized using imidazolium-based organic structure-directing agents, fluoride anions and germanium and silicon as tetrahedral components of the framework. Due to the presence of stacking disorder, the structure elucidation of HPM-14 was challenging, and different techniques were necessary to clarify the details of the structure and to understand the nature of the disorder. The structure has been solved by three-dimensional electron diffraction technique (3D ED) and consists of an intergrowth of two polymorphs possessing a three-dimensional channel system, including an extra-large pore opened through windows made up of sixteen tetrahedral atoms (16-membered ring, 16MR) as well as two additional sets of odd-membered (9MR) and small (8MR) pores. The intergrowth has been studied by scanning transmission electron microscopy (C s -STEM) and powder X-ray diffraction simulations (DIFFaX), which show a large predominance of the monoclinic polymorph A.<br>