Polytype formation

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

The main causes of structural defect and foreign polytype formation are related to the imperfection of chemically etched surfaces and to the surface contamination. At a high... 

Impurity elements that were shown to have the most pronounced influence on polytype formation are those in Groups 111 and V of the PSE-that is, electron acceptors (Al, B, etc.) and electron donors (N, P). Aluminum was first noted to stabihze the 4H structure and, since this primary observation, other investigations have noted the same effect The same stabilizing effect is also attributed to boron. 

AN Pilyankevich, VF Britun, GS Oleynik. Micro-structural studies of polytype formation in oxygen containing aluminum nitride. J Mat Sci 25 3517 (1990). 

The properties of siHcon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal siHcon carbide. The pressureless-sintered siHcon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded siHcon carbide refractories. Table 1 Hsts the properties of the hiUy compacted, high purity material. 

However, SiC also exhibits other stacking sequences, as shown in 6.15.6., given on the next page. These arranged layers are called "polytypes" and are prevalent where simple compounds such as SiC and SiN are involved. In many cases, the properties of such compounds depend, to a large extent, upon the specific stacked layers obtained during formation. 

The familiar diamond structure, with each atom covalently bonded in a perfect tetrahedral fashion to its four neighbors, is adopted not only by C but also by Si and Ge. Silicon can also adopt a wurtzite structure (see below), an example of a polytype (one of several crystal structures possible for a substance having an identical chemical composition but differing in the stacking of layers, and which may exist in a metastable state after its formation at some different temperature or pressure). 

Synthesis of zeolite Y in the presence of Gd(III) complexes of 18-crown-6 resulted not only in the encapsulation of the complex but the complex also served as a template for EMT polytype zeolite Y (Fig. 22b) (86). Feijen et al. described how the two different polytypes (the cubic FAU and the hexagonal EMT) can be formed (87). In the absence of an organic template, the FAU structure will form. If Na" "-18-crown-6 is present, it can be absorbed on the surface of the growing zeolite layer. This will influence the interconnection of the layers and, therefore, in the presence of this crown ether, the formation of the EMT framework may be favored. The difference between the pore window sizes is that in the EMT there are two different types 7.3 x 7.3 A in the [001] direction and 7.5 x 6.5 A perpendicular to the [001] direction. (The FAU has pore windows with 7.4 x 7.4 A in the [111] direction.)... 

Evaluation of equilibria 5.144 and 5.145 is complicated by the formation of polytype IM, which is metastable with respect to 2ML The IM 2Ml transforma-... 

In the various intergrowth systems examined (see Table 5.3) there is no evidence for the presence of point defects. The origin of long-range periodicity in the complex recurrent intergrowth systems is, however, intriguing. The importance of elastic forces in the formation of polytypes, shear structures and infinitely adaptive structures was... 

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al.,... 

With these ideas in mind, we used Aerosil to prepare a silica-richer ZSM-20. Surprisingly, under our reference conditions, we observed the formation of another open structure, here called "FAU-polytype", that could not be identified by XRD to any of the known faujasite-type polymotphs. "Figure 10 . The doublet of the... 

A series of preliminary results have suggested that synthesis temperatures higher than 100°C favour the formation of zeolite Beta with respect to ZSM-20 (32). We have therefore tried to further improve the synthesis conditions of zeolite Beta by heating the hydrogels that contained the optimized TEA/AI2O3 ratios (equivalent to samples 2,4 and 5, Table VI) at higher temperatures. A selected set of results are shown in Table VII and compared with the corresponding reference conditions that yielded ZSM-20, FAU-polytype and zeolite Beta at 100°C. 

The hexagonal polytypes are obtained as mixtures, although impurities encourage the formation of certain structures e.g., boron produces the 6H structure, A1 gives 4H and La encourages the formation of 2H structures. 

One of the central themes in chemistry is to understand how atoms and molecules react together to form the products of any chemical reaction. This is true of all reactions of solids, and one of the most fascinating aspects of the phases described in the previous Sections is how they form and how they react chemically. This is especially so of the polytypes, where mechanisms for the formation of these complex structures are being sought, and the occurrence of such materials remains an enigma yet to be solved. 

Banfield JF, Bailey SW (1996) Evidenee for formation of regularly interstratified serpentine-ehlorite minerals by tetrahedral inversion in long-period serpentine polytypes. Am Mineral 81 79-91 Banfield JF, Barker WW (1998) Low-temperature alteration in tuffs from Yucea Mountain, Nevada. Clays Clay Minerals 46 27-37... 

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