F-CENTER ELECTRON QUANTIZATION IN ALKALI HALIDE CRYSTALS Background Alkali halides and alkaline earth halides are strongly ionic compounds

Alkali halides and alkaline earth halides are strongly ionic compounds that solidify as crystals. The name for the alkali elements (Li, Na, K, Rb and Cs) comes from the Arabic word alqili (ashes of the salt-wort plant that was once used as a source of sodium and potassium for making glass from sand). The name for the halogen elements (F, Cl, Br, I—and the rare radioactive element astatine) comes from the Greek word halos (ocean) that recalls their dissolved presence (as ions) in seawater. Most alkali halides (LiF through CsF; see Table 1) crystallize in the face-centered cubic rocksalt structure with monovalent ions (cations, anions) having well-defined ion radii (Table 2). Because of cation/anion radius-ratio packing issues, CsCl, CsBr and CsI instead crystallize in the simple-cubic CsCl structure. The alkaline earth fluorides CaF2, SrF2 and BaF2 crystallize in the cubic fluorite structure. Ionic radii depend somewhat on coordination. In the rocksalt structure, each ion is 6-coordinated by ions of the opposite charge; in the CsCl structure each ion is 4-coordinated by its opposite ion. Cations in the fluorite structure are 8-coordinated by anions; anions are 4-coordinated by cations.