Electronic paradoxes in the structures of minerals
For the majority of readers of this chapter, perhaps the traditional view of solids, and especially of minerals, is the one usually described as the ‘ionic’ model. On this scheme solids are assembled by packing together charged billiard ball-like ions of different sizes, and described by the rules collected together by Pauling (1929). Such a viewpoint has the advantage that it leads to an easy visualization based upon packing concepts learned since infancy. In more quantitative language the’ size’ of the ion concerned is set by the form of the repulsive part of the potential between it and other ions, which prohibits a mutual approach less than a certain distance. In recent years this conceptually simple model has been substantially improved by the inclusion of more sophisticated potentials between atoms, derived either empirically or from quantum chemical calculations on small molecules. Some of the results which have been obtained are quite impressive and have led to a useful route with which to model solids, from the study of structural stability fields to the determination of the compressibility of quartz, for example. In spite of the increase in sophistication it is still, of course, a ball-and-spring model. The basic ideas behind such a model are very different from those behind the quantum-mechanical-based models. These dominate chemical theory and are employed by the physicist who wishes to predict parameters such as the critical pressure for the NaCl→CsCl transition in rocksalt, or the critical superconducting temperature in NbGe3.
KeywordsHigh Occupied Molecular Orbital Lower Unoccupied Molecular Orbital Orbital Model Jahn Teller Zone Edge
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