Defects in Oxides

When HRTEM is used for examining nanoparticles of oxides, in which the proportion of surface area greatly increases, most structural information concerns the surface. For example, HRTEM images of core-shell quantum dots can show the shell structure and its thickness directly. HRTEM images of metal oxide nanotubes can also be regarded as surface profile images. The appHcation of TEM in nanomaterials will be further discussed below. 

General speaking, a crystal defect is a type of microstructure in which the parent crystal structure is locally disturbed. Such a microstructure carmot be described by a superstructure, even by an incommensurate superstructure. Since the defect areas are usually very small, they may not be detected by XRD and neutron diffraction methods. HRTEM is the most powerful technique to detect them and to investigate their relation with the parent crystals. The defects can greatly change the properties of an oxide. Some examples of the common defects in oxides are given in the following sub-sections. 

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From the formation reaction of protonic defects in oxides (eq 23), it is evident that protonic defects coexist with oxide ion vacancies, where the ratio of their concentrations is dependent on temperature and water partial pressure. The formation of protonic defects actually requires the uptake of water from the environment and the transport of water within the oxide lattice. Of course, water does not diffuse as such, but rather, as a result of the ambipolar diffusion of protonic defects (OH and oxide ion vacancies (V ). Assuming ideal behavior of the involved defects (an activity coefficient of unity) the chemical (Tick s) diffusion coefficient of water is... 

The question raised by Anderson (1970,1971) and Anderson et al (1973) as to whether anion point defects are eliminated completely by the creation of extended CS plane defects, is a very important one. This is because anion point defects can be hardly eliminated totally because apart from statistical thermodynamics considerations they must be involved in diffusion process. Oxygen isotope exchange experiments indeed suggest that oxygen diffuses readily by vacancy mechanism. In many oxides it is difficult to compare small anion deficiency with the extent of extended defects and in doped complex oxides there is a very real discrepancy between the area of CS plane present which defines the number of oxygen sites eliminated and the oxygen deficit in the sample (Anderson 1970, Anderson et al 1973). We attempt to address these issues and elucidate the role of anion point defects in oxides in oxidation catalysis (chapter 3). 

We thus ask What causes CS planes to nucleate (i.e. what are the reasons for anion vacancy aggregation and collapse in an oxide catalyst) and grow. We examine the response of defects in oxidizing atmospheres and, in particular, the role of anion vacancy concentrations in catalytic oxides. The EM results have led to novel concepts in oxidation catalysis (Gai 1981, 1992-1993, Gai et al 1982). 

These studies have led to a new understanding of me fundamental defect processes that occur at me oxide catalyst s surface and of the role of extended CS plane defects in oxide catalysts. Furmermore, me studies suggest mat anion point... 

Bogomolova, L. D., Stefanovsky, S. V., Teplyakov, Y. G. Dmitriev, S. A. 1997. Formation of paramagnetic defects in oxide glasses during the bompardment of their surface with charged particles. Materials Research Society Symposium Proceedings, 465, 657-664. 

II. The Mechanism of the Reaction of Gases WITH Semiconducting Crystals 1. The Electron Defects in Oxides Influenced by Gases... 

G5703A tRNAAsn Severe defects in oxidative phosphorylation HI... 

Peroxy entities may indeed represent an ubiquitous type of defects in oxides and silicates, both natural and synthetic. The reason is that traces of dissolved "water" are everywhere. In optical waveguide fibers made of low-OH fused silica a dramatic increase of Si-OH and attendant loss of transmittance have been reported upon exposure to H2 probably by way of the reaction Si/°° Si + H2 2 Si-OH (15,35). ... 

In the 1960s, all the tools needed to treat hydrogen defects in oxides [41-43] were fundamentally developed by Wagner and collaborators. However, up to the end of the 1970s, real developments in the field did not take place. During the last years of this decade, some significant studies were carried out [44-48], After that, it was realized that the introduction of defects in some perovskite structures determine the protonic conductivity of these materials. In this regard, Iwahara and... 

High Blood Alanine Level Associated with Defects in Oxidative Phosphorylation Most individuals with genetic defects in oxidative phosphorylation are found to have relatively high concentrations of alanine in their blood. Explain this in biochemical terms. 

One of the challenges for the future is to refine existing analytical techniques and to develop new ones for characterizing catalysts and species adsorbed on catalyst surfaces. Of particular need are methods that allow the observation of a catalyst under actual working conditions, because the structure and composition of a catalyst surface in the working environment are often different from those existing prior to reaction. Examples of such effects include the reconstruction of metal surfaces, the appearance of defects in oxides, and the deposition of poisons. 

Gianfranco P. Ab initio theory of point defects in oxide materials structure, properties, chemical reactivity. Solid State Sci. 2000 2 161-79. 

Other factors are temperature, ionic strength, rate of flow of corrosive fluid, etc. At pH >11 corrosion rate decreases due to the formation of a protective film of ferric hydroxide/oxide. Although general corrosion rate decreases above this pH, the metal becomes susceptible to intercrystalline attack (at defects in oxide film) and thus fails due to caustic embrittlement. 

While the effective g value is expressed in terms of three principal values directed along three axes or directions in a single crystal, only the principal values of g can be extracted from the powder spectrum rather than the principal directions of the tensor with respect to the molecular axes. (Therefore it is more correct to label the observed g values as gi, g2, g3 rather than g gyy, in a powder sample.) In the simplest case, an isotropic g tensor can be observed, such that all three principal axes of the paramagnetic center are identical (x = y = z and therefore gi= gi = g-i). In this case, only a single EPR line would be observed (in the absence of any hyperfine interaction). With the exception of certain point defects in oxides and the presence of signals from conduction electrons, such high symmetry cases are rarely encountered in studies of oxides and surfaces. 

Often the most important properties of materials are directly or indirectly connected to the presence of defects and in particular of point defects [18]. These centers determine the optical, electronic and transport properties of the material and usually dominate the chemistry of its surface. A detailed understanding and a control at atomistic level of the nature (and concentration) of point defects in oxides is therefore of fundamental importance to synthesize new materials with well defined properties. This has lead in recent years to the birth of the new field of defect engineering. Of course, before to be created in controlled conditions point defects have to be known in all aspects of their physico-chemical properties. The accurate theoretical description of the electronic structure of point defects in oxides is essential for the understanding of their structure-properties relationship. 

I am indebted to A. M. Ferrari (Torino), T. Bredow (Hannover), C. Sousa and N. Lopez (Barcelona), L. Giordano, R. Soave and D. Ricci (Milano) for their substantial help in the study of defects in oxides. The cooperation with the groups of F. Bias (Barcelona), E. Giamello and C. Pisani (Turin), N. Rbsch (Munich), and U. Heiz (Losanne) has been invaluable to elucidate many open questions and to link experimental with theoretical results. Financial support through the INFM Projects PAIS and PRA-ISADORA is gratefolly acknowledged. 

J. Nowotny, Surface segregation of defects in oxide ceramics materials. Solid State Ionics, 28-30 (1988) 1235-1243. 

FIGURE 10-36 Cytoplasmic inheritance of the petite mutation in yeast. Petite-strain mitochondria are defective in oxidative phosphorylation owing to a deletion in mtDNA. 

Mikkelsen J (1984) Self-diffusivity of network oxygen in vitreous Si02. Appl Phys Lett 45 1187-1189 Millot F, Niu Y (1997) Discussion of O in Fc304 An experimental approach to study the behavior of minority defects in oxides. J Phys Chem Solids 58 63-72 Millot F, Picard C (1988) Oxygen self-diffusion in non-stoichiometric rutile Ti02-x at high temperature. Solid State Ionics 28-30 1344-1348... 

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