Isolation of Defects

Figure 13.16 The J-Vdevice performances of PIDT-PhanQtPCyiBM cells following isolation of defects using a razor blade.

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

In recent studies on perfused rats hearts (Veitch et al., 1992), it was found that differences in the sensitivity of complexes 1-lV to ischaemic damage were dependent upon the duration of ischaemia and the presence of oxygen. The demonstration that complex 1 is a major defective site dependent upon isolation of mitochondria from homogenates of the tissue by in vitro methods seemed important to us. We therefore decided to attempt to make noninvasive measurements of mitochondrial function soon after reperfusion in transplanted rabbit kidneys by surface fluorescence (for mitochondrial NADH levels) and near infra-red spectroscopy (NIRS) for the redox state of cytaas. 

Williams, F. M. and W. F. Flintoff. Isolation of a human cDNA that complements a mutant hamster cell defective in methotrexate uptake. J. Biol. Chem. 1995, 270, 2987-2992. 

All disorders except those in group 5 are due to defects of nDNA and are transmitted by Mendelian inheritance. Disorders of the respiratory chain can be due to defects of nDNA or mtDNA. Usually, mutations of nDNA cause isolated, severe defects of individual respiratory complexes, whereas mutations in mtDNA or defects of intergenomic communication cause variably severe, multiple deficiencies of respiratory chain complexes. The description that follows is based on the biochemical classification. 

Although this estimate of the interaction energy between defects is simplistic, it demonstrates that a fair number of defects may cluster together rather than remain as isolated point defects, provided, of course, that they can diffuse through the crystal. It is difficult, experimentally, to determine the absolute numbers of point defects present in a crystal, and doubly so to determine the percentage that might be associated rather than separate. It is in both of these areas that theoretical calculations are able to bear fruit. 

Even when the composition range of a nonstoichiometric phase remains small, complex defect structures can occur. Both atomistic simulations and quantum mechanical calculations suggest that point defects tend to cluster. In many systems isolated point defects have been replaced by aggregates of point defects with a well-defined structure. These materials therefore contain a population of volume defects. 

Although this is correct in one sense, isolated iron vacancies appear not to occur over much of the composition range. Instead, small groups of atoms and vacancies aggregate into a variety of defect clusters, which are distributed throughout the wustite matrix (Fig. 4.6). The confirmation of the stability of these clusters compared to isolated point defects was one of the early successes of atomistic simulation techniques. 

As in the case of wiistite, one of the earliest applications of atomistic simulations was to explore the likely stability of this defect cluster. It was found that not only the 2 2 2 arrangement but also other cluster geometries were preferred over isolated point defects. 

The origin of defects causing discoloration in polyolefin pellets can be identified using light and electron microscopy. For example, PE pellets from an in-plant recycle re-pelletlzing process contained pellets that were off color and had black specks, as shown In Fig. 11.6(a). One of these defects was isolated using the cross sectioning technique, as shown in Fig. 11.6(b). The cross section revealed an intense reddish particle that caused the discoloration of the pellet. 

Defects can be calculated in supercells that are multiples of the unit cell for the underlying undefected structure (e.g., 7-Lii/2Mn02). If the supercell is large enough, the periodic images of the defect will have negligible interaction, giving an approximation of an isolated lone defect. 

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