Modular structures composition variation

In the previous sections composition variation has been attributed more or less to point defects and extensions of the point defect concept. In this section structures that can be considered to be built from slabs of one or more parent structures are described. They are frequently found in mineral specimens, and the piecemeal way in which early examples were discovered has led to a number of more or less synonymic terms for their description, including intergrowth phases, composite structures, polysynthetic twinned phases, polysomatic phases, and tropochemical cell-twinned phases. In general, they are all considered to be modular structures. 

Modular structures can be built from slabs of the same or different compositions, the slabs widths can be random or regular, and the slabs themselves can be ordered in a variety of ways (Fig. 4.19). Composition variation can then occur by variation of the amount of each slab type present, by variation in the degree of order present, or, when slabs have the same composition, by changes in atom ratios introduced at the slab boundaries. In addition, the planar boundaries that divide up a modular structure create new coordination polyhedra in the vicinity of the fault that are not present in the parent structure. These may provide sites for novel chemical reactions or introduce changes in the physical properties of significance compared to those of the parent structure. 

As these two examples show, besides altered unit cell dimensions, a series of modular structures may or may not show a regular variation of composition. When only one slab type is present this will depend upon the nature of the planar boundaries involved. However, a change of composition must occur if the series of... 

Gross stoichiometry variations in these modular systems are often acconunodated by the formation of other series members or the incorporation of planar faults, often equivalent to isolated lamellae of other series members, into the structure. However, many individual phases also show composition variation which is sometimes accommodated by random populations of point defects and in other phases by ordering and the generation of new structures. In both cases composition flexibility is important in influencing the physical and chemical properties of these materials. In particular this effect has been well studied in the cuprate superconductors, where changes in composition have repercussions for the superconducting transition temperature, r, of the phase. 

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