Defective Systems

Extended defects, on the other hand, correspond to structural imperfections in the assembly of either lattice planes (planar defects), as stacking faults in layer structures, or lattice directions (linear defects), as dislocations. 

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The description theoretical study of defects frequently refers to some computation of defect electronic structure i.e., a solution of the Schrodin-ger equation (Pantelides, 1978 Bachelet, 1986). The goal of such calculations is normally to complement or guide the corresponding experimental study so that the defect is either properly identified or otherwise better understood. Frequently, the experimental study suffices to identify the basic structure of the defect this is particularly true when the system is EPR (electron paramagnetic resonance) active. However, if the computational method properly simulates the defect, we are provided with a wealth of additional information that can be used to reveal some of the more basic and general features of many-electron defect systems and defect reactions. 

The electronic-structure calculations reported by computational theorists have contributed greatly to the understanding of defects in semiconductors, both in terms of characterizing specific defect systems and in revealing... 

For the simple two-defect system introduced in the preceding section it is found that... 

Ocular complications following local or systemic administration of steroids include glaucoma, cataracts, adverse influence on specific ocular infections, pseudotumor cerebri, ptosis, mydriasis, subjective visual complaints, visual field defects, systemic absorption of the topical medication, conjunctival and palpebral petechiae, epithelial punctate keratitis, and, possibly, corneal and scleromalacia. ... 

Stem cell therapy is aimed at replacing, supplementing, repairing or reprogramming defective systems or cells in patients with immune system... 

In materials that have high ionic conductivity, effects such as the above are undoubtedly very important. They show up particularly in materials that have a high concentration of mobile ions and in experimental values of the ac conductivity measured as a function of frequency. In materials with a high carrier concentration, mobile ions are inevitably quite close together, separated by at most a few angstroms. Consequently, ions cannot hop in isolation but are influenced by the distribution of mobile ions in their vicinity. This contrasts with the behaviour of dilute defect systems with low carrier concentrations. In these, the mobile ions are well separated from each other and their conduction can largely be treated in terms of isolated hops. 

Employee following personal example set by his supervisor, due to a defective system for establishing and maintaining supervisory performance standards... 

Trivial examples of metastability are solid solutions. Because these are inherently defect systems, they cannot be thermodynamically stable at low temperatures. Most of our high Tc superconductors need to be regarded as solid solutions which are then necessarily metastable phases. We could dismiss this as an irrelevant observation on the basis that solid solutions are merely required in order to adjust the carrier concentration to appropriate levels. However, we seem unable to generally make stable high Tc superconductors. One could even suggest that there is a correlation between Tc and metastability the higher the Tc, the more unstable. 

In what follows we present a generalization of [45,46] by introducing two subbands to distinguish the role of hot and cold regions of the defect-system spectrum, cf. [43],... 

Figure 1 3 1. Energetic characteristics of a model cuprate on the doping scale. 1 - the underdoped state small pseudogap As 2 - the large pseudogap A, 3 - the defect system superconducting gap Aa 4 - the itinerant system superconducting gap Ay 5 - Tc. The insert shows normal state gaps. p = 0.18 Pp- 0.12 0 0.23 p(Tcm) = 0.23]...

Stretched polyacetylene with very low sp3 defect concentration has much higher conductivity and anisotropy than other materials. This point emphasizes the importance of oriented, low-defect systems if charge transport is principally along the conjugated chains. This discussion illustrates that no complete, consistent theory of transport in PA has yet been developed. 

In the enumeration of the types of defect in 1.2 some effort was made to produce a rational classification. The placing of a defect solid in one or other class is undoubtedly somewhat arbitrary, particularly as one solid of definite composition may well contain defect systems of different kinds and these may interact. Moreover, some types of defect have temperature-dependent concentrations and would disappear in the true thermodynamic equilibrium state at 0 i they may be referred to as thermal, indicating their origin. Defects of other kinds that are inherent in the particular solid would be present in the equilibrium state at they have been referred to as biographical defects. Defects of all types... 

The only way to rationalize the transport data for nanocrystalline ionic materials is to consider the systems separately, from the viewpoint of the level of defects in the crystals and the nature ofthe samples. In the case of lowly defective systems, such as the alkaline earth fluorides, there is good evidence for a conductivity enhancement, and the data can be explained in terms of models based on the space-charge layer. A key experiment here was the observation of enhanced conductivity in very thin alternating Cafb/Bafb layers when measured perpendicular to the layers [298]. This was explained as being due to the space-charge layers overlapping and saturating the layers. However, this observation is difficult to explain in terms of a model based on surface mismatch. 

The treatment of the defective lattice follows the customary two-region approach (Catlow and Mackrodt, 1982 this volume Chapter 7) in which the total energy of the defective system is minimized by variation of the nuclear positions (and shell displacements) around the defect. The crystal is partitioned into an inner region, immediately surrounding the defect where the relaxation is assumed to be greatest, and a less perturbed outer region. In the inner region the... 

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