Polytypes related

Ara] Arakcheeva, A.V., Karpinskii, O.G., Polytypic Relations in the Structures of the Group of Hexagonal Ferrites. I. Ca Ferrites of the Structural Type of Ca2Fei6025 , Sov. Phys. Crystal-logr., 33(3), 378-380 (1988) (Experimental, Crys. Structure, 5)... 

Staeking faults and sometimes proper polytypism are found in many inorganic compounds - to pick out just a few, zinc sulphide, zinc oxide, beryllium oxide. Interest in these faults arises from the present-day focus on electron theory of phase stability, and on eomputer simulation of lattice faults of all kinds investigators are attempting to relate staeking-fault concentration on various measurable character-isties of the compounds in question, such as ionicity , and thereby to cast light on the eleetronic strueture and phase stability of the two rival structures that give rise to the faults. 

Although it does not relate to polytype identification, we mention a 14N static and MAS-NMR study of WZ AIN, which revealed a fortuitously small Cq < 10 kHz [167]. For comparison, the 27A1 Cq in the same compound was measured to be 1.913 MHz, and had an unusual positive temperature coefficient that has been seen for other metal nuclei in WZ structures. The 14N MAS-NMR spectrum of BN yielded a Cq of 142 kHz corresponding to an axially-symmetric tensor in the hexagonal ring structure [167]. 

Electronic properties of the disulphides of the fourth, fifth and sixth group transiton metals crystallizing in the Cdl2(lT), MoS2(2H, 3R) and related polytypic structures have been rationalized by similar considerations. Covalent interaction between quadrivalent cations and sulphide ions is so strong that collective d states are formed in all the cases. The properties crucially depend on the electronic configuration and the... 

Brown and Bailey (1962) examined 300 chlorites from different localities and found that approximately 80% had the lib structure but found examples of the orthorhombic lb, monoclinic lb and la structural type. The relative abundance of the polytypes was related to structural stability. Composition influences to some extent the stability of the chlorites through its effect on the cation charge and amount of distortion of the hexagonal network caused by size adjustments. Increasing tetrahedral A1 substitution is accompanied by an increase in octahedral Fe to maintain a reasonable degree of fit between the two types of layers. 

Ca2Fe205 and oxides of the CaMn03 v family are good examples of such vacancy-ordered structures. Complex intergrowth phases (ordered as well as disordered) involving brownmillerite and other related phases are commonly found in some of the anion-deficient oxides. Some of them also show polytypism due to different modes of stacking of the hexagonal and cubic layers. 

Figure 10.16 (a) HRTEM of an edge of a zeolite P crystallite, annotated to show the stacking directions in different parts. Nanodomains related to polytypes A and B are indicated. Defects are visible in the center of the image, where domains with different stacking directions meet (b) Fourier-averaged image of a domain of type B, with structural... 

Stacking faults are a-boundaries for which a = 2xg R. (0gi-0g2) is zero for all g. In some structures, stacking faults and twins are closely related, and different regular sequences of these defects produce various polytypes. Wollastonite is a relatively simple example of such a structure, for which the stacking faults have been studied in some detail by TEM, both by their a-fringe contrast and in two-dimensional high-resolution lattice images. 

Members of families of closely related structures, the formation of which h dependent on the growth mechanism of the crystals, are termed polytypes. The) are not normal polymorphs, and are formed only by compounds with certain type of structure. The best-known examples are SiC, Cdl2, ZnS, and certain comple oxides, notably ferrites, to which reference should be made for further details. 

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