Ionic crystals color centers

So far, we have dealt with optically active centers based on dopant ions, which are generally introduced during crystal growth. Other typical optically active centers are associated with inhinsic lattice defects. These defects may be electrons or holes associated with vacancies or interstitials in ionic crystals, such as the alkali halide matrices. These centers are nsually called color centers, as they prodnce coloration in the perfect colorless crystals. 

If the coupling of the electrons to certain centers is strong, their spectra may be distinguished from that of the crystal as a whole (point defect color centers in ionic crystals, polarons in semiconductors). The spectra of defects can therefore be used for analytical or even kinetic investigations. In principle, it should be possible to construct devices which have, under favorable conditions, a sufficient spatial resolution to experimentally determine the basic kinetic quantity c,( , t). 

A color center or F-center is formed from diffusion of a small quantity of M+ ion into an ionic crystal MX. Since the crystal must keep its charge neutrality, additional electrons readily move to fill the vacancies normally occupied by anions. Thus the composition of the crystal becomes (M+)i+i(X e ). The origin of the color is due to electronic motion, and a simple picture of an electron in a vacancy is illustrated by the particle in a three-dimensional box problem, which is discussed in Section 1.5.2. 

Salts of the bases MOH are crystalline, ionic solids, colorless except where the anion is colored. For the alkali metal ions the energies required to excite electrons to the lowest available empty orbitals could be supplied only by quanta far out in the vacuum ultraviolet (the transition 5p6 —5p56s in Cs+ occurs at 1000 A). However, colored crystals of compounds such as NaCl are sometimes encountered. Color arises from the presence in the lattice of holes and free electrons, called color centers, and such chromophoric disturbances can be produced by irradiation of the crystals with X rays and nuclear radiation. The color results from transitions of the electrons between energy levels in the holes in which they are trapped. These electrons behave in principle similarly to those in solvent cages in the liquid ammonia solutions, but the energy levels are differently spaced and consequently the colors are different and variable. Small excesses of metal atoms produce similar effects, since these atoms form M+ ions and electrons that occupy holes where anions would be in a perfect crystal. 

Irradiation of ionic crystals results in atomic and electronic dislocations. The trapping of displaced electrons by anion vacancies results in the absorption of visible and near ultraviolet light, which give these crystals their characteristic colors. These pseudoatomic electrons and their vacancies are called color centers. 

Color centers are simple point defects in crystal lattices, consisting of one or more electrons trapped at an ionic... 

The calculations of real systems (for example, color centers in ionic crystals, [475]) were made based on the model potential of Abarenkov and Heine [476] ... 

---