Shear mechanism, crystallographic

Anion vacancy in perovskites is more common than cation vacancy. Unlike the well-known case of W03, anion-deficient nonstoichiometry is not accommodated by the crystallographic shear mechanism, but by assimilation of vacancies into the structure, resulting in supercells of the basic network. The review by Rao et al. (24) contains numerous examples of this kind of behavior. Anion excess has been described in a more limited number of systems. Structural details of this type of compounds can be found in Rao et al. (24) and Smyth (25). 

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. 

Erom the preceding discussion, the distinction between misfit defects shear domains formed by pure shear and CS planes formed by the elimination of anion vacancies in a specific crystallographic plane by shear and the collapse of the oxide lattice on that plane can be understood. This distinction between defects is central to catalytic reaction mechanisms in oxides. However, it is often not made in the literature on oxide catalysis and solid state oxide chemistry. This can result in an incorrect interpretation of observed data and of the role played by lattice oxygen atoms in catalytic reactions. The former are regions containing... 

The nonstoichiometry may be found on the oxygen sites as is the case with stabilized zirconia (Yj Zri j )02 j/2, metal deficient oxides such as Fei j 0, anion excess oxides such as UO2+J and systems that display combinations of these effects. The vacancies in such systems may cluster or ultimately may order by some mechanism such as crystallographic shear. Catlow has discussed such systems in detail. ... 

The cooperative movement of large numbers of atoms represents an alternative, and in some ways more precise [83], mechanism of reaction in addition to the well-established interface advance and diffusion-controlled processes which are considered throughout this book. Examination of the possible participation of crystallographic shear in the reactions of solids has been largely restricted to refractory oxides, but comparable or related behaviour could, in principle, operate in a variety of other solid state rate processes. 

The reduction occurs by direct oxygen removal from the solid oxides (solid-state diffusion). The basic underlying mechanism is not known (diffusion of O, OH, H2O) and is likely to vary for different for different phase transitions. On the final reduction the metal remains pseudomorphous to the starting oxide, forming a polycrystalline metal sponge. Solid-state reactions are characteristic for low reduction temperatures (<750 °C) and the early WO3 - WO2 9 transition ( crystallographic shear transition). 

There are various operation modes for piezoelectric sensors, depending on the crystallographic orientation of the plate and the material [1]. These modes include transversal compression, thickness or longitudinal compression, thickness shear action and face shear action. Also available are piezoelectric polymeric films, which are very thin, lightweight and pliant, such as polyvinylidene fluoride (PVDF) [3,4]. These films can be cut easily and adapted to uneven surfaces. Resonance applications are not possible with PVDFs because of their low mechanical quality factor. However, they can be used in acoustical broad-band applications for microphones and loudspeakers. 

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