Alkali halides band structure

The alkali halides cire noted for their propensity to form color-centers. It has been found that the peak of the band changes as the size of the cation in the alkali halides increases. There appears to be an inverse relation between the size of the cation (actually, the polarizability of the cation) and the peak energy of the absorption band. These are the two types of electronic defects that are found in ciystcds, namely positive "holes" and negative "electrons", and their presence in the structure is related to the fact that the lattice tends to become charge-compensated, depending upon the type of defect present. 

These spectra exist, characteristically, of bands which may be very broad or very narrow, and which may show vibrational structure. Examples of very broad bands without any structure at all, not even at very low temperatures, are found in the spectra of the F centre (an electron trapped at a halide vacancy in the alkali halides), and the tungstate (WO2-) group in CaW04. The spectral width may approach a value of 1 eV, and the Stokes shift of the emission band may be 2 eV. 

These processes give rise to the electronic absorption bands of lowest energy observed in the pure undamaged single crystals which occur at 7.68 eV for MgO and 6.8 eV for CaO (142). Defects within the crystal structure are associated with optical absorption bands at reduced energies [for example, the anion vacancy band in the alkali halides (143)] because of the lower Madelung potential. The energy is still absorbed by the processes described in Eqs. (27) and (28), but the exciton is now bound to a defect and is equivalent to an excited electronic state of the defect. 

Poole, Liesegang, Leckey, and Jenkin (1975) have reviewed published band calculations for the alkali halides and tabulated the corresponding parameters obtained by various methods. Pantclidcs (1975c) has used an empirical LCAO method that is similar to that described for cesium chloride in Chapter 2 (see Fig. 2-2), to obtain a universal one-parameter form for the upper valence bands in the rocksalt structure. This study did not assume only one important interatomic matrix clement, as we did in Chapter 2, but assumed that all interatomic matrix elements scale as d with universal parameters. Thus it follows that all systems would have bands of exactly the same form but of varying scale. That form is shown in Fig. 14-2. Rocksalt and zincblende have the same Brillouin Zone and symmetry lines, which were shown in Fig. 3.6. The total band width was given by... 

The application of the salt/molecule reaction technique to the study of reactions with Lewis bases such as H2O and NH3 presents the possibility for a different type of interaction which may find some cinalogy in transition metal coordination chemistry. The structure of small complexes such as MX H20 are of considerable interest both experimentally and theoretically. These studies were initiated as a result of the observation of several beinds in the spectrum of alkali halide salts in argon which could not readily be assigned to the isolated salt species. Rather, it was shown that these bands were due to reaction of the salt with impurity H2O, which was always present in these experiments to some degree. A study was then initiated to investigate these beinds, and the nature of the reaction conplex. 

For the alkali metal halides the influence of the anion is practically non-existent as can be seen from the comparison of RbCl, RbBr and Rbl (Table 3). In the more covalent structures both atoms appear to be effected by increase in pressure, as changes in the band structure lead to an increase in conductivity. 

Radiation-induced decomposition of insulating solids has been the subject of extensive research for many years. Because of their structural simplicity, the alkali and silver halides have perhaps received the widest attention. Studies of radiation-induced decomposition in azides could represent the next logical step in structural complexity. The azides in many respects are similar to the halides. Like the alkali halides, the alkali azides are primarily ionically bonded with band gaps of the order of 8 eV. Like the halides, there are azides with smaller band gaps (less than 4 eV). Important differences between the halides and azides are the presence of the triatomic azide anion and the lattice symmetry differences, which are perhaps a result of the nonspherical charge distribution on the azide ion. The salient questions which arise for the purpose of this chapter when one compares the azides to the hahdes are How does the the presence of the molecular anion influence radiation-induced decomposition are new and/or different kinds of defects produced how does the azide molecular anion influence the defect production process ... 

The only defects found in the azides which have counterparts in the alkali halides are the F and FJ centers (the F center consists of an electron in an anion vacancy the FJ center is an electron occupying two adjacent anion vacancies). ESR of the F center was observed by Carlson et al. [17] and by King et al. [18] in sodium azide which had been UV-irradiated at 77°K. The observed spectrum consists of 19 hyperfine lines due to the interaction of an electron trapped in an azide vacancy with the nuclear spins (/ = 3/2) of the six nearest-neighbor sodium ions. The ESR signal is correlated with an optical absorption band by thermal and optical bleaching (see below). Bartram et al. [19] have performed calculations of the wave-functions for the F center in sodium azide. Their predictions of the expected hyperfine structure and optical absorptions are in good agreement with experiment. 

An important distinction between the alkali azides and ionic materials, such as the halides, is the susceptibility of the azides to point-defect production by UV radiation. There has been little research concerned with understanding how UV radiation produces individual point defects in the azides. Clearly the presence of the molecular anion, NJ, is important, and many of the defects are dissociation products of the azide ion. However, just the presence of a molecular anion is not sufficient. Cyanates, for example, do not have defects produced in them by UV light even though the NCO is isoelectronic with N3 [35]. It is important to consider the detailed electronic structure of the azide ion as well as the electronic structure of the lattice. Such factors as the energy of the first excited state with respect to the ground state, the proximity of an unbound state of the NJ to the lower excited state, the electron affinity and the ionization potential, as well as the band structures of individual azides, are important. 

We use a periodic supercell model based on the large unit cell (LUC) method [22] which is free from the limitations of different cluster models applicable mainly to ionic solids, e.g., alkali halides. The main computational equations for calculating the total energy of the crystal within the framework of the LUC have been given in Refs. [22-24]. Here, we shall outline some key elements of the method. The basic idea of the LUC is in computing the electronic structure of the unit cell extended in a special manner at k = 0 in the reduced Brillouin zone (BZ), which is equivalent to a band structure calculation at those BZ k points, which transform to the reduced BZ center on extending the unit cell [22]. The total energy of the crystal is... 

Several factors have an impact on the appearance of the infrared spectra of inorganic compounds. The crystal form of the compound needs to be considered. Crystalline lattice bands manifest themselves in the far-infrared region and changes to the crystal structure will be observed in the spectra. The consequence is that non-destructive sampling techniques are preferred for such samples. Techniques such as alkali halide discs or mulls can produce pressure-induced shifts in the infrared bands of such materials. 

Color centers in alkali halide crystals are based on a halide ion vacancy in the crystal lattice of rock-salt structure (Fig. 5.76). If a single electron is trapped at such a vacancy, its energy levels result in new absorption lines in the visible spectrum, broadened to bands by the interaction with phonons. Since these visible absorption bands, which are caused by the trapped electrons and which are absent in the spectrum of the ideal crystal lattice, make the crystal appear colored, these imperfections in the lattice are called F-centers (from the German word Farbe for color) [5.138]. These F-centers have very small oscillator strengths for electronic transitions, therefore they are not suited as active laser materials. 

Nickolov, Z. S., and Miller, J. D. 2005. Water structure in aqueous solutions of alkali halide salts FTIR spectroscopy of the OD stretching band. J. Colloid Interface Sci. 287 572. 

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