Shear planes structure

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. 

Topographically resolved scanning tunnelling spectroscopy confirms that the occupied electronic states are associated with the trough like defects, which are therefore believed to correspond to shear-plane structures intersecting the surface (fig. 15). Metal-metal bonding between adjacent W ions within the shear planes pushes the states deep down into the gap [268,270,271]. 

Shear-plane structures in real systems are invariably more complex. Figure 2 gives a still partially simplified illustration of the shear plane in Ti02-x- However, the essential features of the schematic plane discussed above apply to real systems that is, shear-plane formation eliminates point defects by a change in the mode of linking of MOe octahedra. 

Long-range periodicity based on extended defects is not, however, confined to shear-plane structures. Indeed the occurrence of extended defect super-lattices is widespread. The adaptive structures discussed by Anderson have already been referred to in the Introduction. A further illustration of the phenomenon, which strikingly illustrates its generality, is provided by the void lattice observed in certain irradiated metals, e.g., Mo, where voids, typically of diameters 50 A, formed by the aggregation of irradiation induced vacancies, order to give a stable f.c.c. lattice in which the voids are separated by 300 A. 

Stoneham and Durham s theory thus appears to be as satisfactory as its generality allows. Two recent developments promise greater specificity. The first (Iguchi and Tilley follows directly from Stoneham and Durham s work. However, the calculations in these studies were based on the real <102> shear-plane structure in VO3 x rather than the hypothetical <100> plane used in Stoneham and Durham s work. Furthermore, information from microscopy studies was used to quantify the magnitude of the defect forces F. The approach is promising it has supported Stoneham and Durham s conclusion that elastic interactions are sufficient to lead to the observed ordering. As yet, however, details of the interaction function have not been worked out. 

Schematic illustration of shear-plane formation. Structure (a) with aligned oxygen vacancies shears to eliminate these vacancies in favour of an extended planar defect in the cation lattice as in (b). % cations oxygen ions are at the mesh intersections...

Fig. 21. (a) The nature of the glide shear plane defects in three-dimensional projection and (b) in one layer of idealized structure, showing the novel glide shear process and the formation of glide shear plane defects. Filled circles are anion vacancies, (c) Schematic of glide shear. Glide defects accommodate the misfit at the interface between catalyst surface layers with anion vacancies (filled circles) and the underlying bulk (85,89). 

An interaction potential between the surface and ions may also be needed in simulating counterion diffusion for the smectite and mica surface models. The form of such an interaction potential remains to be determined. This may not pose a significant problem, since recent evidence (40) suggests that over 98% of the cations near smectite surfaces lie within the shear plane. For specifically adsorbed cations such as potassium or calcium, the surface-ion interactions can also be neglected if it is assumed that cation diffusion contributes little to the water structure. In simulating the interaction potential between counterions and interfacial water, a water-ion interaction potential similar to those already developed for MD simulations (41-43) could be specified. 

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. 

Operation (7) The mother structure (for example, TiO2 (rutile)) is divided into blocks with the dimension of a = x (n is an integer), where is a shear plane with plane indices (hkl) and is the plane spacing of H. (Fig. 2.2(a)). 

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