Planar intergrowth

Modular structures are those that can be considered to be built from slabs of one or more parent structures. Slabs can be sections from just one parent phase, as in many perovskite-related structures and CS phases, or they can come from two or more parent structures, as in the mica-pyroxene intergrowths. Some of these crystals possess enormous unit cells, of some hundreds of nanometers in length. In many materials the slab thicknesses may vary widely, in which case the slab boundaries will not fall on a regular lattice and form planar defects. 

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. 

The crystal structures of many minerals can be visualized as ordered intergrowths of two or more structurally distinct planar units. Thompson... 

Besides these intergrowths, the crystals also contain a variety of faults. These can take the form of isolated wrong lamellae in an ordered sequence, totally disordered regions between two ordered regions, or planar boundaries between ordered regions. Some examples are shown in Figure 21. We will not describe these in detail here, but most boundaries will be associated with stoicheiometric variability. 

All of these materials seem to be equally intolerant of point-defect populations as those described earlier, and others could readily be cited. Thus the conclusion to be drawn from this Section is that phases which accommodate changes in anion to cation stoicheiometry by way of planar fault or intergrowth behaviour comprise a substantial number of inorganic materials, and such phases are in no way the poor relations of defect chemistry compared to systems which are point-defect biased. 

We can finally conclude that the number of chemical systems which appear to reject point-defect populations as a mode of accommodating their non-stoicheiometric behaviour is large and varied and here we have touched upon only a few which make use of planar faults or parallel lamellar or foliar intergrowth structures. The results presented show that physical terms, such as elastic strain, are of importance in controlling the microstructures of such phases, but whether they form or whether they coexist with some form of point-defect clusters may well depend in a sensitive way to the anion-cation bonding within the individual co-ordination polyhedra which made up the structure. The continuing research in this area is certain to produce new and unexpected results before complete answers to the problems posed here are found. 

Figure 8 (a)-The sheet anion topology in theM (U02)5(V0J205 compounds as an intergrowth between uranophane and P-UbO sheets. The orientations of the VO4 tetrahedra populating triangular sites lead to two different isomers (b and d) and to corrugated (c) or planar (e) layers for M = Na, K, P-Rb and M = a-Rb, respectively. 

Cation vacancies and interstitials, (111) twins and stacking faults, grain boundaries, microstrains, misfit dislocation network at C03O4/C0O interface Dislocations and (100) stacking faults intergrowth of e and P phases. Cations vacancies and superstructure (110) stacking faults and twins Clusters of point defects (110) twins surface steps, dislocations, spinel microinclusions, planar defects stabilized by impurities. 

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