Alkali halides, excess electrons

COLOR CENTERS. Certain crystals, such as the alkali halides, can be colored by the introduction of excess alkali metal into the lattice, or by irradiation with x-rays, energetic electrons, etc. Thus sodium chloride acquires a yellow color and potassium chloride a blue-violet color. The absorption spectra of such crystals have definite absorption bands throughout the ultraviolet, visible and near-infrared regions. The term color center is applied to special electronic configurations in the solid. The simplest and best understood of these color centers is the F center. Color centers are basically lattice defects that absorb light. 

Ochsenfeld C, Gauss J, Ahlrichs R. An ab initio treatment of the electronic absorption spectra of excess-electron alkali halide clusters Na , , CI up to Na18Cl17. J Chem Phys 1995 103 7401-7407. 

A divalent atom such as calcium substituted for a monovalent cation in alkali halides releases two electrons and becomes a doubly charged ion. The new atom of calcium has excess positive charge which can couple with a negative defect, for example an alkali metal vacancy or an interstitial halide, to create a dipole. 

GouiaryBS, Adrian FJ (1960) Wave functions for electron-excess color centers in alkali halide crystals. Solid State Physics 10 127-247... 

In Section 2.3 the structural and optical properties of neutral and cationic Na clusters at r = 0 K as functions of size are presented and compared with experimental data recorded at low temperature. The temperature-dependent line-broadening will be illustrated by the example of Na9, since in this case a comparison with experimental data at different temperatures is particularly instructive. In Section 2.4 the results of ab initio molecular dynamics (AIMD) studies on Li9 will serve to show different temperature behavior of distinct types of structures as well as their isomerization mechanisms. The study of possible metal-insulator transitions and segregation into metallic and ionic parts in finite systems carried out on prototypes of nonstoichiometric alkali halide and alkali hydride clusters with single and multiple excess electrons is presented in Section 2.5. A comparison of structural and optical characteristics of Na F and lAnUm (n > m) series allows us to illustrate the influence of different bonding properties. 

The ground-state properties of nonstoichiometrie X Y clusters (X = Na, Li, K and Y = Cl, F) with single and multiple excess electrons have been extensively studied experimentally [41-43] and theoretically [44-46] since they are good candidates for possible metal-insulator transitions and metallic-ionic segregation in finite systems. Hydrogenation of lithium clusters has been also investigated [47, 48]. It is of interest to establish similarities and differences among properties of alkali halide and alkali hydride clusters, since both bulk materials have a common structure but the electron affinities of F and H atoms are very different (3.4 versus 0.75 eV). The question can be raised to what extent these differences are reflected in properties of small finite systems. 

Perhaps the most widely studied imperfection phenomenon is that of color centers induced in alkali halide crystals by a -irradia-tion or by introducing excess alkali metal. The best known of these is the F center which is now widely accepted to consist of a halide ion vacancy with an electron trapped in it. As yet there has been no definitive treatment of this center from first principles. The usual zeroth-order treatment approximates the solution as a particle in the box giving (l/l ). Using F-peak energies and lattice parameters for several alkali halides Mollwo showed that this is an approximately correct relationship. A more detailed analysis of the data by Ivey gave a relationship of the form... 

For alkali halide clusters it was first proposed to classify species corresponding to (i) cuboid or deformed sections of the rock salt lattice with the localized excess electrons in the vacancy (surface F-centers), for which relatively high IPs were measured and a strong absorption in the visible-infrared region, in analogy with the bulk color centers, is expected (ii) filled cuboids with excess electrons highly delocalized... 

Excess electrons in clusters also exhibit the above structural characteristics as well as Other quantum mechanical effects that arise when the electron wavelength is comparable to the cluster diameter. Path integral studies of an excess electrons in alkali halide clusters have established the existence of internal cavity structures and of extended surface states. Dissociative attachment, leading to the formation of an isolated alkali atom, and structural isomerization have also been observed. Similar calculations have demonstrated the prevalence of surface states in water clusters. " Representative structures are shown in Figure 5. 

Coloured sodium chloride crystals are due to the formation of non-stoichiometric vacancies in the anion lattice. This vacancy is capable of trapping an electron, which can then move between a number of quantized levels. These transitions occur in the visible region and generate the yellow colour. This type of vacancy in the anion sublattice of an alkali metal halide is called a Farbenzcentre or F centre. F centres can be generated by irradiation to ionize the anion, or by exposure of the lattice to excess alkali-metal cation vapour. Both procedures result in more alkali-metal cations than halide anions in the lattice. 

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