Crystals intergrowth

The occurrence of intergrowths of zeolite Y and ZSM-20, the cubic and hexagonal forms, is analogous to similar intergrowths in SiC and ZnS crystals. Intergrowths in zeolite Y and ZSM-20 do not block channels, as is the case in the erionite-offretite family, where rotation of cancrinite layers blocks the 12MR channels, but are more like intergrowths in the ZSM-5/ZSM-11 family, which modify the channel system. 

Even if the problems of poor crystal intergrowth due to local exhaustion of reactants in the autoclave and synthesis of zeolite material in the bulk of the solution were solved, an important problem remains, related to the fact that several batch synthesis cycles (with their associated heating and cooling processes) are often required to achieve a zeolite membrane of good quality. Thus, a synthesis procedure in which reactants are continuously supplied to the synthesis vessel while this is maintained at a constant temperature would clearly be desirable not only for performance but also for the feasibility of the scale-up. This type of approaches has already been tested for inner MFI and NaA zeolite membranes [33-35], and the results obtained indicate that the formation of concomitant phases and the amount of crystals forming in the liquid phase are greatly reduced. Similarly, the continuous seeding of tubular supports by cross-flow filtration of aqueous suspensions [36-37] has been carried out for zeolite NaA membrane preparation. 

Zoning and internal structure—note differences in zonal birefringence and refractive index (on broken crystals) twinning or crystal intergrowths number of sets of lamellae lamellar width variations. 

Let us consider the various plasma processes for producing powders. One process involves the reaction between metal halides and reactive gases, which have been widely used in ordinary CVD processes. The compositions of the products are controlled linearly by those of the reactants (6). Titanium diboride powder consisting of submicrometer-sized crystals of equiaxed and tabular forms with only small amounts of crystal intergrowth was produced in an arc plasma... 

Definition An intergrowth of two or more macroscopic individuals of the same crystal species is a twin, if the orientation relations between the individuals conform to crystallographic laws. The individuals are called twin partners, twin components or twin domains. 

On the intergrowth structure of zeolite crystals as revealed by wide field and confocal fluorescence microscopy of the template removal processes... 

Figure 3. Exploded and normal representation of the proposed intergrowth structures of the zeolite crystals under study a) CrAPO-5 (front subunits are not shown) b) SAPO-34 c) SAPO-5 (front subunits are not shown) and d) ZSM-5.

Unlike crystals that are packed with identical unit cells in 3D space, aperiodic crystals lack such units. So far, aperiodic crystals include not only quasiperiodic crystals, but also crystals in which incommensurable modulations or intergrowth structures (or composites) occur [14], That is to say, quasiperiodicity is only one of the aperiodicities. So what is quasiperiodicity Simply speaking, a structure is classified to be quasiperiodic if it is aperiodic and exhibits self-similarity upon inflation and deflation by tau (x = 1.618, the golden mean). By this, one recognizes the fact that objects with perfect fivefold symmetry can exist in the 3D space however, no 3D space groups are available to build or to interpret such structures. 

Modular structures are those that can be considered to be built from slabs of one or more parent structures. Slabs can be sections from just one parent phase, as in many perovskite-related structures and CS phases, or they can come from two or more parent structures, as in the mica-pyroxene intergrowths. Some of these crystals possess enormous unit cells, of some hundreds of nanometers in length. In many materials the slab thicknesses may vary widely, in which case the slab boundaries will not fall on a regular lattice and form planar defects. 

These few examples illustrate the range of habits exhibited by minerals whose basic structural units are single chains. In addition, the pyroxenes, which have compositions quite similar to those of the amphiholes, are often found intimately intergrown with the latter double-chain minerals in what appears to be a single large crystal. An intergrowth of amphibole and pyroxene as a fiber, or in a fibrous aggregate, has not yet been described, but it is a likely occurrence. 

Hematites grown hy forced hydrolysis of acidic Fe " solutions at elevated temperatures also show a range of crystal morphologies. The type of anion, the acidity and the presence of additives appear to be main factors that influence the shape (Matijevic Scheiner, 1978 Kandori et al., 1991 Bailey et al., 1993). Additives appear to act by adsorbing on specific planes of the growing crystal. At close to 100 °C, rhombohedral crystals, 50-100 nm in size, sometimes showing intergrowths (stepped appearance)... 

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