Crystallographic free diameters

Zeolite structures are designated by a three capital-letter code, for example, FAU stands for the faujasite structure, to which the well-known X and Y zeolites belong. A very useful short notation is used for the description of the pore system(s) each pore network is characterized by the channel directions, the number of atoms (in bold type) in the apertures, the crystallographic free diameter of the apermre (in A), asterisks (1, 2, or 3) indicating whether the systems is one-, two-, or three-dimensional. To completely specify the pore system, the eventual presence of cages (or channel intersections) should be indicated, along with their... 

Mordenite is a naturally-occurring silica-rich zeolite (Si/Al 5) which also can be synthesized readily (34). Meier determined the structure of its fully hydrated sodium form (28). The mordenite framework is characterized by a micropore system composed essentially of parallel elliptical cylinders of maximum and minimum crystallographic free diameters of 7.0 and 5.7A, respectively, these main channels being interconnected by smaller side channels. 

The most intriguing feature of the stmctures of compounds 1 and 2 is that they contain isolated porous nanotubules formed by comer sharing of pentagonal bipyramids and Se04 tetrahedra (Fig. 3a). Such nanometer-scale tubules formed by two types of coordination polyhedra are new in the realm of inorganic oxosalts probably, the most closely related but yet distinct are elliptic porous nanorods in the stmcture of yuksporite, a natural material from the Kola peninsula, Russia [17], The uranyl selenate nanotubules in the stmctures of 1 and 2 have circular cross-sections with outer diameters of 17 and 25 A (= 1.7 and 2.5 nm), respectively. The crystallographic free diameters of the tubules are 4.7 and 12.7 A for 1 and 2, respectively. 

The figure captions include information on the channels (CH). shorthand notation has been used for the description of the channels the various frameworks. Each system of equivalent channels has been characterized by the channel direction, the number of tetrahedra forming the smallest rings of the channels, and the crystallographic free diameters... 

Crystallographic free diameters depend on the state and the composition of the zeolite. The channel dimensions of isotoypic species can differ appreciably, particularly in the case of nonrigid frameworks. 

Selection of Zeolites. The selected zeolites consisted of the 2 principal crystallographic types—i.e., mordenite and faujasite—which provide an interesting range of intracrystalline cage volumes of 0.29 and 0.54 cm per cm zeolite, respectively net charges per surface oxygen of 0.083 and 0.214 e units, respectively and free cavity diameters of about 9 and 11.8 A, respectively (3). 

Zeolites can be classified in many ways. Two convenient methods are on the basis of pore size and chemical composition, that is, the Si/Al ratio. The pore diameter is determined by the size of the free apertures in the structure, which is dependent on the number of T atoms (T = Si or Al) that form the aperture. Table 10.1 summarizes some examples of zeolites based on pore size classification. It should be noted that the values typically reported in the literature are determined by crystallographic studies. While these numbers are good guides, it is important to note that the actual pore size depends on many factors, including temperature, firamework composition, and the type of extra-framework cations present in the zeolite. These factors can lead to subtle changes in effective pore sizes and subsequently large changes in material properties (adsorption/reactivity). 

The residual strains in an aluminum film, 1 pan in thickness, on a Si substrate, 550 //m thick and 200 mm in diameter, were studied by x-ray diffraction. It was found that the d-spacing of crystallographic planes in the film, which were oriented parallel to the plane of the film, decreased by 0.054%, compared to the situation when the film was stress-free. 

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