Complex defect structure

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 In many systems isolated point defects have been replaced by aggregates of point defects with a well-defined structure.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. 

As in the previous chapter, most work has been carried out on oxides, and these figure prominently here. As the literature on oxides alone is not only vast but is also rapidly increasing, this chapter focuses upon a number of representative structure types to explain the broad principles upon which the defect chemistry depends. However, despite considerable research, the defect chemistry and physics of doped crystals is still open to considerable uncertainty, and even well-investigated simple oxides such as lithium-doped nickel oxide, Li Nij- O, appear to have more complex defect structures than thought some years ago. 

Metal hexacyanoruthenates possess a lower symmetry. Several compounds have highly disordered structures, especially when no alkali cations are present for charge compensation. Such a complex defect structure has been found for a completely potassium free Prussian blue precipitated very slowly from a solution in concentrated hydrochloric acid [25, 26]. Here, the structure still remains cubic face-centered however, one-third of the [M1 -1(CN)6] is vacant, randomly distributed and that space is filled with water molecules. The coordination sphere of the remaining ions is maintained... 

Metal hexacyanoruthenates possess a lower symmetry. Several compounds have highly disordered structures, especially when no alkali cations are present for charge compensation. Such a complex defect structure has been found for a completely potassium free Prussian blue precipitated very slowly from a solution in concentrated hydrochloric acid [25, 26]. Here, the structure still remains cubic face-centered however, one-third of the [M - (CN)(, is vacant, randomly distributed and that space is filled with water molecules. The coordination sphere of the remaining m1 -1 ions is maintained unchanged however, the mean coordination sphere of the M ions is decreased (mW(nC)4.5(H2O)i.5). No iron ions occupy interstitial positions, that is, only two types of iron environments exist. Since that special kind of Prussian blue has been the first and hitherto only Prussian blue that could be obtained as sufficiently large crystals to perform a single crystal structure analysis, practically all textbooks, and later publications present that defect structure as the real structure of Prussian blue, completely forgetting that this defect structure is an extreme that forms... 

Inorganic phases with broad composition ranges are called grossly nonstoichiometric phases, and considerable effort has been put into the clarification of the structure of these materials. The initial model for such materials is to suppose that they contain high populations of vacancies or interstitials, to make up the compositional imbalance. A few examples of this approach, from among the many in the literature, are chosen as illustrative. It is possible that the examples cited will be shown to have more complex defect structures in the future. 

During the same period, it also became apparent that the method could be used to provide qualitative guidance as to the nature of complex defect structures in heavily doped and nonstoichiometric materials. One of the best examples was the study [12] of defect aggregation in nonstoichiometric wustite (Fei xO) - a problem that still remains controversial. This study, however, established the stability of the defect aggregate shown in Fig. 2, in which four vacancies surround a central Fe " interstitial site moreover it suggested ways in which these 4 1 clusters could aggregate. 

This approach can be implemented in Excel spreadsheets, as done for a number of complex defect structures of ternary oxides by F.W. Poulsen and others. 

Radical polymerization is often the preferred mechanism for forming polymers and most commercial polymer materials involve radical chemistry at some stage of their production cycle. From both economic and practical viewpoints, the advantages of radical over other forms of polymerization arc many (Chapter 1). However, one of the often-cited "problems" with radical polymerization is a perceived lack of control over the process the inability to precisely control molecular weight and distribution, limited capacity to make complex architectures and the range of undefined defect structures and other forms of "structure irregularity" that may be present in polymers prepared by this mechanism. Much research has been directed at providing answers for problems of this nature. In this, and in the subsequent chapter, we detail the current status of the efforts to redress these issues. In this chapter, wc focus on how to achieve control by appropriate selection of the reaction conditions in conventional radical polymerization. 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

This reveals that two alternative defect structures can be imagined, one with free holes and one with Ni3+ defects. A further possibility is that the hole may be lightly bound to an Ni2+ ion to give a defect complex that could be written (NiNi + h ). All of these descriptions are valid. The one adopted would be the one most consistent with the measured properties of the solid. 

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... 

This indicates that the defect structure is complex and may vary with degree of doping. Further studies are needed to clarify the defect structure of this notionally simple solid. 

In practice, the defect structure of the materials LiJCo, M)02 and Lix(Ni, M)02 under oxidizing conditions found at cathodes, is complex. For example, oxidation of Fe3+ substituted lithium nickelate, LL(Ni, Fe)02, under cathodic conditions leads to the formation of Fe4+ and Ni4+. Conductivity can then take place by means of rapid charge hopping between Fe3+, Ni3+, Fe4+, and Ni4+, giving average charges of Fe3+S and Ni3+S. These solids are the subject of ongoing research. 

However, a detailed model for the defect structure is probably considerably more complex than that predicted by the ideal, dilute solution model. For higher-defect concentration (e.g., more than 1%) the defect structure would involve association of defects with formation of defect complexes and clusters and formation of shear structures or microdomains with ordered defect. The thermodynamics, defect structure, and charge transfer in doped LaCo03 have been reviewed recently [84],... 

The results for the apparent radius of STM images for individual states can be used to interpret experimental images directly. For surfaces with complex periodic structures, such as Si(lll)-7 X 7 and Si(lll)-5 X 5, the concept of imaging individual atomic states is a much better description than surface Bloch functions. For adatoms and defects, the individual state description is the only possible one. 

However, structural chemistry in oxides with large departures from stoichiometry cannot be solely explained by point defects. Defects cannot remain isolated and interactions between them begin to occur. Our discussion therefore begins with a general description of the fundamental knowledge about nonstoichiometry in oxides. Understanding such disorder and the complex defect... 

Anion-deficient fluorite oxides are also present, for example, U02- c, Ce02-x The presence of anion vacancies in reduced fluorites has been confirmed by diffraction studies. In reduced ceria for example, some well-ordered phases has been reported (Sharma et al 1999). The defective compounds show very high anion mobilities and are useful as conductors and as catalytic materials as will be described later. However, the structures of many anion-deficient fluorite oxides remain unknown because of the shear complexity of the disordered phases. There are, therefore, many opportunities for EM studies to obtain a better understanding of the defect structures and properties of these complex materials which are used in catalysis. 

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