Some Applications for Defect Chemistry

Before we proceed to analyze defect reactions by a mathematical approach, let us consider two applications of solid state chemistry. We begin with a description of some phosphor defect chemistry. 

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LaMnOs is a prototype for a wide range of rare earth oxomanganates whose physical properties can be radically altered by partial substitution of alkaline earths for La or by similar replacement of Mn by a 3d or other small to medium sized cation. Lai j,Aj Mn03 (A = Ca, Sr, Ba) constitutes one of the more interesting systems for CMR applications. Pure stoichiometric LaMnOs is an insulator that orders antiferromagnetically at 135 K and it is only by introducing a critical amount of Mn+ into the stmcture that the CMR effect can be observed. Thus, some couuneut about the nature of the defect chemistry of LaMnOs is in order. 

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. 

The theoretical results have also indicated that when metal atoms are bound to specific defects their chemical activity may change, in particular can increase. This is likely to be true also for small metal clusters. This has not been fully appreciated so far. In fact, even inert supports, like silica, alumina, or magnesia, can interact strongly with the supported metal if this is bound at a defect site and can have a direct role in the chemistry of the supported species. Some preliminary calculations on supported clusters, however, suggest that the effect of the defect on the cluster electronic structure is restricted to very small, really nanometric clusters of about ten atoms size [224]. Should the size of active catalysts in real applications go down to this size, the specific interaction with the substrate could no longer be ignored in the interpretation of the catalytic activity. 

An important simplification results if we can consider the bonding between atoms to be a local phenomenon. In this event, we would need to consider only the immediate neighbours of the adsorbate or defect atoms, and we arrive at the cluster models circled in Fig. 1. Of course, some properties of the system will depend on its extended nature. Others, including the variation in total energy with small displacements of atoms, should be described satisfactorily by a cluster calculation. In such cases, the problem has been reduced to one of molecular dimensions, so that the methods of molecular physics or theoretical chemistry could be used. For many systems of interest to the solid-state physicist, where a typical problem might be the chemisorption of a carbon monoxide molecule on the surface of a ferromagnetic metal surface such as nickel, the methods discussed in much of the rest of the present volume are inappropriate. It is necessary to seek alternatives, and this chapter is concerned with one of them, the density functional (DF) formalism. While the motivation of the solid-state physicist is perhaps different from that of the chemist, the above discussion shows that some of the goals are very similar. Indeed, it is my view that the density functional formalism, which owes much of its development and most of its applications to solid-state physicists, can make a useful contribution to theoretical chemistry. 

The chromate pretreatment layer, which is also called the chromate conversion coating (CCC) varies in thickness depending on the chemistry of the process and the application method used. The CCC layer is, however, usually not thicker than a few microns, which in coating weight is somewhere between 5 and 25 mg/m, expressed as Cr [19], This CCC layer improves the adhesion between the metal and the primer, it aids in the protection of scratches and defects and it also protects cut edges of the metal to some extent [20]. The hexavalent chromate in the CCC layer is known for its low solubility and the self-healing effect, which means lliat it only leaches out on demand when the base metal has been scratched [21]. 

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