Defect interface chemistry

In this and the following chapter, we will describe the most important simple (binary) crystal structures found in ceramic materials. You need to know the structures we have chosen because many other important materials have the same structures and because much of our discussion of point defects, interfaces, and processing will use these materials as illustrations. Some, namely FeSi, TiOi, CuO, and CU2O, are themselves less important materials and you would not be the only ceramist not to know their structure. We include these oxides in this discussion because each one illustrates a special feature that we find in oxides. These structures are just the tip of the topic known as crystal chemistry (or solid-state chemistry) the mineralogist would have to learn these, those in Chapter 7, and many more by heart. In most examples we will mention some applications of the chosen material. 

Chemical properties of solid compounds are not intrinsic but extrinsic as the chemistry of solids is for the most part the kind of chemistry that occurs at the surface or at an interface between solid phases. Interface chemistry is the subject of Chapter 6. Bulk chemistry of solids is linked to the presence of defects. An introduction to defect chemistry in crystals is given in Chapter 10. Insofar as bulk chemistry relates to ion and electron transport it has been discussed above under electrical properties. 

Silicon has been and will most probably continue to be the dominant material in semiconductor technology. Although the defect-free silicon single crystal is one of the best understood systems in materials science, its electrochemistry to many people is still a matter of alchemy. This view is partly a result of the interdisciplinary aspects of the topic Physics meets chemistry at the silicon-electrolyte interface. 

Reaction of solids with halogens has been much less widely studied because it has less application. Studies have tended to be mostly in the realm of pure chemistry and have turned up some curiosities in the behaviour of defects, especially electronic defects. The role of such species as reaction intermediates, especially in transport to and from reaction interfaces, is not very well understood and, in our opinion, is probably generally rather underestimated. Two major obstacles to the study of these species are their transient nature at high temperatures and the absence of detailed information about them in the oxides, because the lack of nuclear spin on 16 O greatly limits the information obtained from the technique, electron spin resonance, which has been most valuable for the halides. 

Muller DA, Mills MJ (1999) Electron microscopy probing the atomic structure and chemistry of grain boundaries, interfaces and defects. Mater Sci Eng A 260 12... 

Often the most important properties of materials are directly or indirectly connected to the presence of defects and in particular of point defects [126,127]. 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 the atomistic level of the nature (and concentration) of point defects in oxides are therefore of fundamental importance also to understand the nature of the metal-oxide interface. The accurate theoretical description of the electronic structure of point defects in oxides is essential for understanding their structure-properties relationship but also for a correct description of the metal-oxide interface and of the early stages of metal deposition on oxide substrates. 

Ongoing investigations into the chemistry of porous silicon surfaces seek to develop methods for the preparation of chemically functional interfaces that protect the underlying silicon nanocrystallites from degradation without changing or annihilating their intrinsic behavior. The native, hydride-terminated surface is only metastable under ambient conditions and oxidation of freshly prepared porous silicon commences within minutes when exposed to air. While surface oxide can suitably passivate the nanocrystalline silicon and stabilize its photoluminescence, the electrically insulating and structurally defective character of this oxide layer... 

One of the most difficult problems for ab initio quantum chemistry is to determine the potential energy function for a chemical reaction on a metal surface. Why is this so First of all, the metal substrate is strongly delocalized. This means that the system cannot be modeled [1] by considering just a small or medium-sized cluster of metal atoms. On the other hand, the band structure techniques that would simplify calculations for a bare metal surface cannot be directly applied because the translational symmetry is broken by the presence of the reactants. As a result one has the difficulty of dealing with extended interactions without the benefit of simplifications due to symmetry. Many problems involving surfaces, interfaces, impurities, or defects in solid state materials fall under this broad rubric along with various solution phenomena as well. 

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