Insulators structure classification

Room temperature Donor conductivity, ort, Scm Metal-insulator 1 transition, K Structural classification (after Morikami et al.127)... 

In previous chapters we focused on physical properties for which electrons are only important in the sense that we must know the ground state of the electrons to understand the material s energy. There is, of course, a long list of physical properties where the details of the electronic structure in the material are of great importance. Two examples of these properties are the classification of a bulk material as a metal, a semiconductor, or an insulator and the existence and characteristics of magnetic properties. In this chapter we examine how information related to these questions can be obtained from DFT calculations. 

Solid-state systems are frequently classified according to their physical, structural or chemical properties. Such schemes are extremely helpful since properties related to any such classification are typically known and facilitate identifying solids with special material classes. The best-known examples of these schemes are conductivity or resistivity measurements by means of which metals are easily distinguishable from insulators. However, frequently clear-cut decisions between material classes are not possible, since anisotropy, chemical composition, binding forces and local effects wash out distinct properties and lead to competition or coexistence. 

There are aspects of the discussion in the last paragraph which can be extended well beyond the compounds covered by the Robins and Day classification. Consider the following problem. NiO, when pure, is a pale solid. TiO and VO both have the same structure as NiO but are black and are almost metallic conductors of electricity, whereas NiO, when pure, is an insulator. That there is some electronic explanation for this pattern is made evident by the observation that when suitably doped with an impurity (Li20 is the one usually cited) NiO also becomes black and highly electrically... 

The anomalous behavior of tin results from the two crystal structures in which it can exist in the solid state. One of these structures is a metal and the other is a semiconductor, and it is therefore necessary to give a brief description of these basic material classifications and the type of atomic bonding that gives rise to these different structures. There are three major classes of solids metals, semiconductors, and insulators [25]. Approximately three-quarters of all naturally occurring elements are metals. Those materials which are neither metals nor nonmetals but that share the characteristics of both include gallium, tin, antimony, and polonium. Examples of elements and compounds for these classes of materials are given in Table 3. These classes of solids result from their characteristic atomic bonding. 

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