Extended defect structures ordered

The Schottky defect population in the electrolyte is rather too low for practical purposes. To overcome this problem the Lil is sometimes doped with Cal2. The extra I- ions extend the Lil structure, and the Ca2+ ions form substitutional impurity defects on sites normally reserved for Li+ ions. The consequence of this is that each Ca2+ ion in Lil will form one cation vacancy over and above those present due to Schottky defects in order to maintain charge neutrality. This can be written... 

Internal boundaries in a crystal, when disordered, form extended defects. However, if the boundaries become ordered, they simply extend the unit cell of the structure and hence are no longer regarded either as boundaries or defects (Fig. 3.20c). In addition, some boundaries can change the composition of a solid locally and, if present in large numbers, can change the macroscopic composition noticeably. When these are ordered, new series of compounds form. Boundaries that do cause significant composition changes are described in Chapter 4. 

There is little doubt that many materials that at present are described as containing ordered arrays of point or extended defects will be successfully described as notionally defect-free modulated structures. For example, the intergrowth Aurivillius phases, described as containing extended planar defects, have recently been described compactly as modulated structures. " The same formalism has been applied to hexagonal perovskite structures and superconducting copper oxides. Others will certainly follow. 

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. 

It is already dear that the equilibrium behavior of materials having gross defects is not predicted by theories which are a simple extension of the classical point defect models. This is because strong interactions between defects in parent structures yield extended defects that become regular structural features of textured intermediate phases, which when ordered leave very low concentrations of point defects. The most studied of these structural features is the Wadsley defect resulting from crystallographic shear in typical... 

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