Triple-defect model

Figure 33 (a) HREM image of a defect where the two rows of lead atoms, which appear as double white rows in the thick part of the crystal (white arrows), disappeared they are replaced by three rows of staggered dots, (b) Drawing of the ide ized structural model the defect would consists of the local replacement of a "PbO-( -PbO" triple layer by a "CUO2-Y-Q1O2" triple layer. The defective layer is marked by a black arrow. 

As strong experimental arguments dst that in B2 alloys non-stoichiometry is accommodated by vacancies (triple defects) on the B-rich side, and antisite defects on the A-rich side (see Section 3.2), several simplified models have been developed to describe this behavior (Neumann, 19 7,1980 Edelin, 1979 Bakker and Van Ommen, 1978 Van Ommen etal., 1981). In these models, it is assumed that N = Ny0=Q. The following relations have been found at low defect concentration ... 

The following ball-and-stick molecular model is a representation of thalidomide, a drug that causes birth defects when taken by expectant mothers but is valuable for its use against leprosy. The lines indicate only the connections between atoms, not whether the bonds are single, double, or triple (red = O, gray = C, blue = N, ivory = H). 

Here the narrow prescription of Chapter 1 is widened to deal with more chemically complex phases, in which the materials may contain mixtures of A, B and X ions as well as chemical defects. In these cases, using an ionic model, it is only necessary that the nominal charges balance to obtain a viable perovskite composition. In many instances these ions are distributed at random over the available sites, but for some simple ratios they can order to form phases with double or triple perovskite-type unit cells. The distribution and valence of these ordered or partly ordered cations and anions are often not totally apparent from difEraction studies, and they are often clarified by use of the bond valence sums derived from experimentally determined bond distances. Information on the bond valence method is given in Appendix A for readers unfamiliar with it Point defects also become significant in these materials. The standard Kroger- fink notation, used for labelling these defects, is outlined in Appendix B. 

Triple resonance (double ENDOR) studies on cis- and trans- P(Ac) [469] have shown that two separate sites are associated with the same paramagnetic center, and that this defect is a highly delocalized 7r-radical interacting with sp carbons and H-atoms bonded thereto, i.e. a trapped soliton. Comparison of the magnitude of the tensor elements with other data, e.g. that from MO calculations, indicates that this 7r-radical is delocalized over ca. 49 C-atoms [444]. That the resonances in both cis- and trans-P(Ac) are due to the same type of defect is evidenced by studies that show that both isomers show similar ENDOR spectra at low temperature, ca. 2 °K tFig. 11 46). Several ENDOR studies of P(Ac) and other CPs [440, 444, 470] provide support to the solitonic (in P(Ac)) and polaronic (in other CPs) models of conduction in CPs. 

Fig. 5.24 The Kossel model of a surface lower dimensional defects within the surface as a two dimensional imperfection. The differing energies of the structural elements with varying numbers of contacts (see numbering) are of particular importance for reactivity and growth [104], The triple contact corresponds to the so-called half-crystal site.

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