ABSTRACT
Ia-VIIb compounds—that is, alkali halides—are prototypical colorless ionic crystals. Their crystal structure is of the rock-salt type except for three compounds (CsCl, CsBr, and CsI) that possess the cesium chloride structure. Melting points are generally in the 620–990°C range, which is relatively low. Although alkali halides are important from the point of view of basic research as mentioned above, their luminescence is rarely utilized in practical applications. This is because alkali halides are water soluble and have low melting points, so that they are unsuitable as hosts of practical phosphors. The range of phenomena has been well elucidated in the literature and are an example of the remarkable contributions that optical spectroscopic studies have made to our understanding of impurity centers. Excitons in all alkali halides except iodides do not move around in the crystal, unlike excitons in IIb-VIb and IIIb-Vb compounds; these excitons are self-trapped immediately after their creation as a result of very strong electron-lattice coupling.
