Crystal structure Polytypic

Crystal Structure. Sihcon carbide may crystalline in the cubic, hexagonal, or rhombohedral stmcture. There is a broad temperature range where these stmctures may form. The hexagonal and rhombohedral stmcture designated as the a-form (noncubic) may crystalline in a large number of polytypes. 

On silicon carbide, it is easier to see and measure step heights than in crystals like beryl, because SiC has polytypes, first discovered by the German crystallog-rapher Baumhauer (1912). The crystal structure is built up of a succession of close-packed layers of identical structure, but stacked on top of each other in alternative ways (Figure 3.24). The simplest kind of SiC simply repeats steps ABCABC, etc., and the step height corresponds to three layers only. Many other stacking sequences... 

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). 

Detailed analysis of different XRD techniques led to the conclusion that CdS used in CdS/CdTe PV cells was polytype, with essentially random stacking of cubic and hexagonal structures in individual crystals [18], This study goes a long way to explaining the wide variation in apparent crystal structure. 

Brown, B.E. and Bailey, S.W., 1963. Chlorite polytypism, 2. Crystal structure of a one-layer Cr-chlorite. Am. Mineralogist, 48 42-61. 

Takeuchi, Y., Nowacki, W. Detailed crystal structure of rhombohedral M0S2 and systematic deduction of possible polytypes of molybdenite. Schweiz. Mineral. Petrog. Mitt. 44, 105—... 

Perhaps the more important questions raised by Rule 5, as Burdett and McLarnan (1984) point out, concern the extent to which it really is borne out by observation. For example, Baur et al. (1983) have developed a numerical index for the degree of parsimony in a crystal structure and have shown that, using this measure, many crystal structures are not parsimonious but lavish in their use of different local environments. Also, the dominance of short-range forces is by no means obvious when ordered structures with extremely large unit cells are observed (e.g., a c dimension of 1500 A in some SiC polytypes Shaffer, 1969). The explanation of such structures poses problems for electrostatic as well as covalent models. 

Takeuchi, Y. and Nowacki, W., Detailed Crystal Structure of Rhombohedral M0S2 and Systematic Deduction of Possible Polytypes of Molybdenite, Schweiz Mineral. Petrographische Mittellungen, 44, 105, (1964). 

This family has many polytypes with a variety of ground state, some of which will be discussed in the following. The crystal structures of these are basically the same, i.e. composed of two-dimensional ET layers, but spatial arrangements of ET molecules in a unit cell are different, which result in drastically different ground states as described below. 

Figure 9.14. A. C MAS NMR spectra of hexagonal SiC polytypes, from Hartman et al. (1987). The spectrum of the cubic (zincblende) structure was not detected by these authors under a wide range of conditions. B. Angultu" dependence of the C NMR lines of single-crystal 6H polytype of SiC. Curve (a) corresponds to the resonance at 21.9 ppm in this crystal, curve (b) corresponds to the resonance at 17.2 ppm and curve (c) to the resonance at 25.4 ppm. From Richardson etal. (1992). Both figures used by permission of the American Chemical Society.

The cubic structure is the dominant crystal structure in both natural and synthetic diamond since the staggered conformation is more stable than the eclipsed due to the slightly lower energy (0.1-0.2 eV per carbon atom). Diamond polytypes and carbyne phases form only during the homogeneous nucleation and growth of diamond powder,... 

In this section, we consider and discuss the structural and chemical features of more than 200 micas. Most are true micas (146 trioctahedral and 55 dioctahedral). Brittle-mica crystal-structure refinements number about twenty, of which only three are dioctahedral (Tables 1-4, at the end of the chapter). Of the six simple polytypes first derived by Smith and Yoder (1956) and reported by Bailey (1984a, p. 7), only five (i.e., IM, 1M, 3T, IM2, and 20) have been found and studied by three-dimensional crystal-structure refinements. 

Most of the trioctahedral tme-mica stmctures are M polytypes and a few are 2Mi, 2M2, and 3T polytypes. In dioctahedral micas, the 2Mi sequence dominates, although 3T and M structures have been found. Brittle mica crystal-structure refinements indicate that the IM polytype is generally trioctahedral whereas the 2Mi polytype is dioctahedral. The 10 structure has been found for the trioctahedral brittle mica, anandite (Giuseppetti and Tadini 1972 Filut et al. 1985) and recently was reported for a phlogopite from Kola Peninsula (Ferraris et al. 2000). The greatest number of the reported structures were refined from single-crystal X-ray diffraction data, with only a few obtained from electron and neutron diffraction experiments. 

Liang J-J, Hawthorne FC, Novak M, Cemy P, Ottolini L (1995) Crystal-structure refinement of boromuscovite polytypes using a coupled Rietvelt-static-stracture energy-minimization method. Can Mineral 33 859-865... 

Takeda H, Haga N, Sadanaga R (1971) Stractmal investigation of a polymorphic transition between 2M2-, lA/-lepidohte and 2Mi-muscovite. Mineral J 6 203-215 Takeda H, Ross M (1975) Mica polytypism Dissimilarities in the crystal structures of coexisting IM and 2M biotite. Am Mineral 60 1030-1040 Takeuchi Y (1965) Stractrrres of brittle micas. Clays Clay Minerals 13 1-25... 

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