Crystallographic shear structures

IR-11.4.5 Defect clusters and use of quasi-chemical equations IR-11.5 Phase nomenclature IR-11.5.1 Introduction IR-11.5.2 Recommended notation IR-11.6 Non-stoichiometric phases IR-11.6.1 Introduction IR-11.6.2 Modulated structures IR-11.6.3 Crystallographic shear structures IR-11.6.4 Unit cell twinning or chemical twinning IR-11.6.5 Infinitely adaptive structures IR-11.6.6 Intercalation compounds IR-11.7 Polymorphism IR-11.7.1 Introduction IR-11.7.2 Use of crystal systems IR-11.8 Final remarks IR-11.9 References... 

A crystallographic shear (CS) plane is a fault in which a plane of atoms has been (notionally) removed from the crystal. In oxides, this is frequently a plane of oxygen atoms, eliminated as a result of reduction. In the resulting structures, the slab types are all identical and the same as the parent phase. To illustrate this phenomenon, crystallographic shear in reduced tungsten trioxide will be described. 

From a structural point of view, Pb2Sr2YCu808 can be oxidized to Pb2Sr2YCu809 with the layers (Cu) becoming (CuO). The formation of the oxidized compound, with a layer (CuO) in its structure, would make possible the existence of compounds with crystallographic shears of the type discussed before in the case of Ba CugOy. No oxidation experiments have been carried out so far, however. No other possible compounds of this homologous series have been reported so far. 

One of the simplest oxides is the rhenium trioxide (ReOs) structure shown in figure lA(b). It consists of an incomplete fee host lattice of with Re in one-quarter of the octahedral sites. (Crystallographic shear (CS) phases (discussed in 1.10.5) based on ReOs may be considered as consisting of the cubic MO2 structure.) Many oxides and fluorides adopt the ReOs structure and are used in catalysis. 

Non-stoichiometric oxides with high levels of disorder may adopt two modes of stabilization aggregation or elimination of point defects. Point defect aggregates forming clusters are examples of the former and extended defect structures like crystallographic shear-plane structures are examples of the latter. 

Nb02.5 and this was described in detail in elegant papers by Anderson (1970, 1973). Ordered phases are based on shear structures, with parallel CS planes (double crystallographic shear) separating the blocks of the ReOs lattice. Ternary and intergrowth block structures have been discovered by extensive HRTEM... 

Crystallographic shear is an elegant structural transformation mechanism in oxides. We now address one of the most fundamental issues in heterogeneous catalysis by oxides the formation and the role of CS planes in oxidation catalysis. 

In these compounds, we find regions of corner-linked octahedra separated from each other by thin regions of a different structure known as the crystallographic shear (CS) planes. The different members of a homologous series are determined by the fixed spacing between the CS planes. The structure of a shear plane is quite difficult to understand, and these structures are usually depicted by the linking of octahedra as described in Chapter 1. 

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