Bulk electronic structure of simple oxides

The first term, called the internal energy in the following, is due to the excess - for the anion - or to the loss - for the cation - of electrons on the outer atomic levels as a result of the charge transfer. The second term, is the covalent energy due to the electron delocalization. In the ionic limit, the internal energy is a maximum while the covalent term is equal to zero. In the covalent limit, it is the other way around. 

This simple example proves that, whenever a charge transfer occurs, at least two terms have to be considered in the total energy the internal energy, sometimes called the charge-transfer energy, and the covalent term. One should also add the terms due to the electron-electron interactions, absent in this non-self-consistent approach, and the short-range repulsion term. At the end of Section 1.4, a more complete approach will be presented. 

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A few moments thought about the nature of the surface of an oxide leads to the conclusion that the surface oxide ion should have quite different properties than the bulk lattice ions. For example, consider a simple cubic oxide such as MO with a sodium chloride structure where each ion is sixfold coordinated if this is cleaved along a <100) plane, then the coordination of the ions in this plane is reduced from six- to fivefold. This new surface will not be ideal, and ions of still lower coordination will also be present where higher index planes are exposed at the surface. However, for MgO prepared by thermal decomposition of the hydroxide or carbonate, evidence from electron microscopy (130) indicates that these have high index planes that... 

W(CN)j /W(CN)j, and Fe3"/Fe2+ in acid solutions [90,91], Simple ETR at sodium tungsten bronzes, NaxW03, with the perovskite structure are fast and are influenced by the sodium bulk content of the electrode as can be seen in Table 2. Unfortunately, the kinetic pattern is not simple because the variation of ETR rate coefficients with sodium content is not the same for each couple [91]. A qualitative interpretation of the ETR kinetic results has been attempted in terms of the density of electronic states at the Fermi level of the oxide electrode [90]. 

Numerous studies have been devoted to bulk oxides, and their properties, both structural and electronic, are now well apprehended. At least, this statement applies to insulating oxides in which correlation effects do not induce a breakdown of the effective one-electron picture. In the remainder of this book, we will mainly focus on these simple oxides, and we will describe how their properties are modified at surfaces, whether they are ideally clean and planar, or contain defects. With the purpose of interpreting these modifications in a unified theoretical framework, we have presented here the basis of a model which stresses the factors governing the electronic structure. These include the relative position of the anions and cations in the periodic table, which fixes the energy difference between the outer levels of the neutral atoms strength of the... 

While the bulk properties of simple binary oxides are well understood, rather little is known about the surfaces of oxides, even the most simple ones. Only recently, if compared with the 30 years of surface science that have passed by [8], researchers have started to study the surface science of oxides. There is a very useful book by V. E. Henrich and P. A. Cox that marks the first milestone in this effort entitled The Surface Science of Oxides [9]. Since the publication of this book, several reviews have appeared that have covered the field up to the present date [10-16]. It is understood that there are classes of technologically very important oxides exhibiting external and internal surfaces, that is, zeolites and mesoporous materials, which will not be discussed here. We refer the reader to a recent article by Thomas et al. [17]. This chapter will treat the properties of single crystalline oxide surfaces in terms of their geometric and electronic structure. 

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