STRUCTURE AND DEFECTS

So far the structure of pure metals has been discussed with reference to bulk characteristics and continuous crystals. However, corrosion is essentially a surface phenomenon and it is necessary to consider how the structure and defects already described interact with free surfaces. At this stage it is convenient to consider only a film-free metal surface, although of course in most corrosion phenomena the presence of surface films is of the utmost importance. Furthermore, it is at free surfaces that the hard sphere model of metals... 

The reaction rates of the thermal reduction and the reoxidation by CO2 are increased by high oxygen anion conductivity and high surface areas. Oxygen anion conductivity is a function of temperature, crystal structure, and defects. Because cycling results in stoichiometric gas-solid reactions, the gas-solid interface can be a crucial parameter depending on the reaction conditions. Whether gas-solid, intraparticle mass transfer, or surface chemical processes are rate-limiting is primarily determined by the reaction temperature and gas flow rates. 

The annihilation characteristics of a positron in a medium is dependent on the overlap of the positron wavefunction with the electron wavefunction [9]. From a measurement of the two photon momentum distribution, information on the electron momentum distribution can be obtained and this forms the basis of extensive studies on electron momentum distribution and Fermi surface of solids [9]. In the presence of defects, in particular, vacancy type defects, positrons are trapped at defects and the resultant annihilation characteristics can be used to characterize the defects [9, 10], Given these inherent strengths of the technique, in the years following the discovery HTSC, a large number of positron annihilation experiments have been carried out [11, 12]. These studies can be broadly classified into three categories (1) Studies on the temperature dependence of annihilation characteristics across Tc, (2) Studies on structure and defect properties and (3) Investigation of the Fermi surface. In this chapter we present an account of these investigations, with focus mainly on the Y 1 2 3 system (for an exhaustive review, see Ref. 11). 

To improve the efficiency of photocatalysts, developments in the future must be based on an understanding of the sophisticated factors that determine the photoactivity of the water-splitting reaction (i) molecular reaction mechanisms involved in the oxidation and reduction of water on photocatalyst surfaces, (ii) structure and defect chemistry of photocatalyst surfaces, and (iii) charge transfer mechanisms between... 

Wachsman, E.D., Ball, G.R., Jiang, N., and Stevenson, D.A., Structural and defect studies in solid oxide electrolytes. Solid State Ionics, 1992, 52, 213-218. 

In summary, as demonstrated within this chapter, HRTEM and associated techniques are very powerful in the study of the microstmctures of oxides and oxide catalysts. The detailed characterization of superstructures, surface structures and defects enriches our knowledge of the structures of oxides and their structure-dependent physico-chemical properties. For characterizing nano-scale oxides, HRTEM plays a particularly important and unique role. Further exciting developments resulting from this fascinating area can be anticipated. 

Although it is now possible to simulate the high-resolution image of a crystal defect of known structure, the calculations are usually very long. Thus, the methods of studying crystal defects (such as dislocations and the more common types of planar defects) discussed in Chapter S, are clearly simpler and more generally appropriate. Nevertheless, the methods of HRTEM have provided information about the structure and defects of certain minerals that could not have been obtained in any other way examples are given later. 

Theoretical Studies of Structure and Defects on Clean Ceria... 

The preceding sections have focused on Che properties of the bulk oxide. However, computer simulation techniques are also well established tools in the study of the structural and defect chemistry of oxide surfaces, which are often difficult to characterize by experiment alone. 

Methods that probe local structure and defects, such as STM and AFM, show that nearly full eoverage on the molecular scale is easy to achieve [36, 39, 40]. Defects for long-chain alkanethiol monolayers are restricted primarily to point defects such as missing single adsorbate molecules, grain boundaries between domains and step edges usually related to discontinuities in the substrate [39, 41]. 

The importance of SPM for the study of monolayers is that it allows the visualization of the structure and defects of transferred and self-assembled monolayers on solid substrates at length scales from <0.1 nm to > 10 pm. It is not fyet ) possible to image monolayers at liquid/fluid Interfaces with SPM techniques. However, it has already been shown that it is feasible to measure interaction forces between a colloidal particle and such interfaces in the presence and absence of monolayers ). 

Libuda J, Winkelmann F, Baumer M, Freund H-J, Bertrams T, Neddermeyer H, et al. (1994). Structure and defects of an ordered alumina film on NiAl(llO). SurfSci, 318, 61... 

The morphology, electronic conductivity, exposed specific surface area, crystal structure, and defect chemistry (cation distribution and oxidation states) of the manganese oxides considerably influence the performance of pseudocapacitor (capacitance, cycle life, and charge/discharge rate).153,156, l57 173,iso, 186,188 ese fea-... 

In summary, it is important to recognise the role of the absorption coefficient a and the electric field x) in the subsurface space-charge region on the activity of the photocatalysts. The latter is closely connected to the surface potential, and thus to the surface structure and defectiveness, and the complexity of the surface of real particles. 

Presently, methods are being developed that focus on the use of superresolution confocal Raman microscopy (CRM), electron back-scattered diffraction (EBSD), and, in combination with high-resolution XRD, to identify and measure the stress distributions of structures and defects that control... 

J. G. Wen, Ph.D. Thesis, TEM study on the structures and defects of Bi-based high Tc superconductor , Institute of Physics, Chinese Academy of Sciences (1991). 

Figure 2.32 (a) Effect of ends of molecules on network structure, (b) Network structure and defects cross-links ° terminus of a molecule (1) elastically active chain (2) inactive loop (3) inactive loose end. An arrow (—>) signifies continuation of the network strucmre. (After Flory, 1944.)... 

In this chapter we have selected a number of case studies to show how atomistic lattice simulations can be used to investigate the structural and defect chemistry of a wide range of high-7 cuprates. Space limitations have necessarily restricted the number of examples to a small range of compounds, chiefly from the La Cu-O, Nd-Cu-O, Y Ba Cu O and Sr-Cu-O systems. 

Fig. 10.2. Schemes of defect structures and defect movements in ice. Small arrows show the actual displacements of protons large arrows show the resulting displacements of the defects.

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