Relations Defect structures

Simultaneous measurements of both o and S in the gas/solid equilibrium for an amphoteric semiconductor (exhibiting an n-p transition) enables the determination of semiconducting properties and the related defect structure based on the Jonker-type analysis. The Jonker analysis involves both S and o measured as a function of equilibrium p(02). However, the Jonker analysis does not require a knowledge of oxygen activity corresponding with the measured electrical properties. Accordingly, the Jonker analysis enables the elimination of any error in the determination of the oxygen activity. 

The recent evidence of workmanship-related defects to major structures in extreme environments leaves a nagging doubt about weather the real issues behind the problems have really been understood, acknowledged and dealt with at all levels. 

I believe, it is fair to state that scanning tunneling microscopy and related techniques such as atomic force microscopy have a tremendeous potential in metal deposition studies. The inherent nature of the deposition process which is strongly influenced by the defect structure of the substrate, providing nucleation centers, requires imaging in real space for a detailed picture of the initial stages. This is possible with an STM, the atomic resolution being an extra bonus which helps to understand these processes on... 

It is not always simple to relate the composition ranges shown on conventional phase diagrams to actual compositions or potential defect structures of a crystal. The way in which this is done is outlined in this section, using the MgO phase range in the Mg0-Al203 system (Fig. 4.3) for illustration. 

There are large numbers of anion excess fluorite-related structures known, a small number of which are listed in Table 4.4. The defect chemistry of these phases is enormously complex, deserving of far more space than can be allocated here. The defect structures can be roughly divided into three categories random interstitials, which in... 

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

There are two identical BR-type sites in the unit cell to accommodate the 1 + x Na+ ions, and there are always vacant BR-type sites in proximity to occupied sites. When cations of higher valence replace sodium, the number of vacant BR-type sites increase in proportion. Although there is no geometrical reason why large cations should occupy other sites, in many compounds, the large cations are located in both BR-type sites and mO sites. As in the case of (3-alumina, the defect structure of each compound is uniquely related to the chemical nature of the cations in the conduction layer. 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

Difficulties in microtomy include the presence of Si, Cl, and sometimes S in the embedding resin which may interfere with the elements under analysis failure to retain the particle within the epoxy and drift of the section with respect to the support grid. Even when these problems are minimized, it requires patience to survey many grids to find an area to analyze that relates to the catalyst surface, pore structure, defect structure, etc. 

Raman spectroscopy is one of the most powerful techniques for the characterization of nanocarbons. It is also a convenient technique because it involves almost no sample preparation and leaves the material unharmed. There are four characteristic bands for CNTs The band at 200 cm-1 is called radial breathing mode (RBM). It depends on the curvature and can be used to calculate the diameter of SWCNTs [61]. The relatively broad D-band at 1340 cm-1 is assigned to sp2-related defects and disorder in the graphitic structure of the material. The tangential C-C stretching mode is located at -1560 cm 1 (G-band). The second order mode of the D-band can be observed (G -band,... 

The Mossbauer effect, although not a substitute for other analytical methods such as x-ray diffraction, can be used to obtain several kinds of structural information about solids. In favorable cases, it is possible to obtain rather detailed information about the electronic configuration of atoms and the local symmetry of their sites by measuring the isomer shift and quadrupole splitting. If more than one valence state of a given atom is present, a semiquantitative determination of the amount of each kind is possible. In solid solutions, the amount of local or long range order can be estimated, and in certain defect structures the relation between the active atoms and the defects can be studied. 

In Chap. C, the thermodynamic and structural outlook of the bond, which had been the matter of discussion in Part A of this chapter, is further developed, and the model formalism, which takes advantage of the well known Friedel s model for d-transition metals but is inspired by the results of refined band calculations, is presented for metals and compounds. Also, a hint is given of the problems which are related to the nonstoichiometry of actinide oxides, such as clustering of defects. Actinide oxides present an almost purely ionic picture nevertheless, covalency is present in considerable extent, and is important for the defect structure. 

The existence of tridymite as a distinct phase of pure crystalline silica has been questioned (42,58—63). According to this view, the only true crystalline phases of pure silica at atmospheric pressure are quartz and a highly ordered three-layer cristobalite having a transition temperature variously estimated from 806 250°C to about 1050°C (50,60). Tridymites are considered to be defect structures in which two-layer sequences predominate. The stability of tridymite as found in natural samples and in fired silica bricks has been attributed to the presence of foreign ions. This view is, however, disputed by those who cite evidence of the formation of tridymite from very pure silicon and water and of the conversion of tridymite M, but not tridymite S, to cristobalite below 1470°C (47). It has been suggested that the phase relations of silica are determined by the purity of the system (42), and that tridymite is not a true form of pure silica but rather a solid solution of mineralizer and silica (63). However, the assumption of the existence of tridymite phases is well established in the technical literature pertinent to practical work. 

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. 

To sum up, the available data relative to oxygen adsorption or catalysis on nickel oxide show the existence of two types of chemisorbed oxygen, one of them being related to the ability of this oxide to accommodate excess oxygen. The evidence concerning this latter property will now be reviewed with emphasis on the defect structure of bulk nickel oxide. 

In recent years inorganic compounds have increasingly been put to practical use, mainly in electric, magnetic, and optical devices. An understanding of the chemical and physical properties of inorganic compounds is indispensable to the progress of materials science and ceramics. Because all inorganic compounds show non-stoichiometry, it is important to understand the nature of non-stoichiometry and the relation between defect structures (i.e. non-stoichiometry) and the properties that they show. 

A closely related disease is caused by a deficiency of propionyl-CoA carboxylase.3 This may be a result of a defective structural gene for one of the two subunits of the enzyme, of a defect in the enzyme that attaches biotin to carboxylases, or of biotinitase, the enzyme that hydrolytically releases biotin from linkage with lysine (Chapter 14). The latter two defects lead to a multiple carboxylase deficiency and to methylmalonyl aciduria as well as ketoacidosis and propionic acidemia. ... 

These traps close to the substrate are related to structural defects, which are generated when the 6T crystallites nucleate on the glass surface. 

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