Sheared phases

Crystallographic shear plane. Series of discrete shear phases are observed among the oxides of several transition metals. By progressive reduction, series of Ti 02 i, V 02 i phases are obtained from the dioxides, as Me 03 i phases may be related to trioxides such as M0O3 or WO3. An example of a crystallographic shear plane formation is schematically shown in Fig. 7.54. 

The literature available and experimental evidence on this subject are not extensive, and much of it concerns the crystallographic shear phases. These of necessity figure prominently in the survey which follows but, in so far as is possible, generalizations to other systems will be made. To some extent, therefore, the present review is a continuation and extension of two earlier review articles which have appeared in this series and which contain much background information to that presented here. 

IR-11.6.1 Introduction Several special problems of nomenclature for non-stoichiometric phases have arisen with the improvements in the precision with which their structures can be determined. Thus, there are references to homologous series, non-commensurate and semi-commensurate structures, Vernier structures, crystallographic shear phases, Wadsley defects, chemical twinned phases, infinitely adaptive phases and modulated structures. Many of the phases that fall into these classes have no observable composition ranges although they have complex structures and formulae an example is Mo17047. These phases, despite their complex formulae, are essentially stoichiometric and possession of a complex formula must not be taken as an indication of a non-stoichiometric compound (cf. Section IR-11.1.2). 

Further reduction with H2, C or CO produces a series of discrete chemical-shear phases (Magn61i phases) of general formula 02 i based on a rutile structure with periodic defects (p. 961), before the black, refractory sesquioxide 203 is reached. Examples are 407, 509, 60u, 7013 and gOi5. The oxides 0, 203 and 30.5 also conform to the general formula V 02 i, but this is a purely formal relation and their structures are not related by chemical-shear to those of the Magneli phases. 

Cellulose and its derivatives can form liquid crystalline solutions in a variety of organic solvents. Most of the lyotropic liquid crystalline phases derived from these compoxmds are cholesteric. Since the flow occurs in a shear field, the chiral nematic structure is transformed into a nematic phase. Nevertheless, shear phase orientation can be destroyed when the applied force is removed. This phenomenon is caused by the driving force that makes the liquid crystal form a supramolecular helical structure with thermodynamic stability [70]. The mesophase has a supramolecular helical structure, whose cellulose molecules are inclined at a small angle, which varies from one layer to another. 

The crystalline material is built from comer-sharing WO octahedra as illustrated in Figure 16.3 this is known as the defect perovskite stracture. The oxide has a propensity to form substoichiometric shear phases (Magneli phases) with some edge-sharing octahedra. The spaces in between the octahedra are large enough to accommodate ions, i.e., the W oxide... 

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