In this chapter we examine the way in which the simple model of an electron in a box can be applied to understand the optical properties of the alkali halides. We then proceed to consider the magnetic properties of electrons which are localized at specific lattice sites. With this discussion of the phenomena of diamagnetism and paramagnetism, we lead up to the next chapter on magnetic order where we see the breakdown of the independent electron approximation giving quite dramatic results.

Point defects in alkali halides

The band gaps of the alkali halides, such as NaCl or KCl, are high and they are found to be excellent insulators. Thus, in the pure and perfect state, the alkali halides are transparent to visible light. However, as a result of either irradiation with X-rays, heating in the vapour of the alkali metal or electrolysis at high temperature, it is possible to create large numbers of negative ion vacancies (Fig. 8.1). These defects correspond to unoccupied lattice sites which would normally contain a halide ion. If the lattice site was simply vacant, the site would appear to be positively charged due to the removal of negative charge on the halide ion. In order to preserve electrical neutrality, it is favourable for an electron to become trapped at this negative ion vacancy. Such an electron trapped at a negative ion vacancy is called an F or colour centre.