Perovskite intergrowth defects

The comparison of the different [m, n] superconducting cuprates suggests that the formation of rock salt or perovskite type defects will not affect dramatically the superconducting properties of these materials. On the opposite, one can expect a dramatic decrease of the superconductivity due to the appearence of double or triple fluorite type defects, since the corresponding regular intergrowths do not superconduct. 

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. 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

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. 

In the K2NiF4 series of phases described above, composition variation can take place by disorder in the width of the perovskite slabs, and the solid contains planar defects. In cases where the components of the intergrowth contain cations with a variable valence, additional defect... 

According to TEM data, perovskites prepared via Pechini route consist of crystalline nano-sized (10-50 nm) particles (Figures 10-12). There are a lot of structural defects and domain boundaries in such samples. This resembles the domain structure of disordered complex Sr-Ba-Fe-Co-(Zr) -0 perovskites prepared via MA route (Fig. 13) [60]. For all these samples but those containing Zr, a cubic structure was observed, while particles were comprised of disorderly stacked microdomains. The BaFeo4Coo4Zro20y sample particles were comprised of the coherent intergrowth of domains with hexagonal BaFeOs-x [JCPDS 23-1024] and monoclinic Ba2Fc205 [JCPDS 43-0256] structmes. 

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