Alkali halide crystals structure

Turning next to an ionic crystal, where the ions may be regarded as spheres, the total volume of the crystal is equal to the volumes of these spheres, together with the appropriate amount of void space between the spheres. To take the simplest case, it is convenient to discuss a set of substances, all of which have the same crystalline structure—for example, the 17 alkali halide crystals that have the NaCl structure. 

The elucidation of the factors determining the relative stability of alternative crystalline structures of a substance would be of the greatest significance in the development of the theory of the solid state. Why, for example, do some of the alkali halides crystallize with the sodium chloride structure and some with the cesium chloride structure Why does titanium dioxide under different conditions assume the different structures of rutile, brookite and anatase Why does aluminum fluosilicate, AljSiCV F2, crystallize with the structure of topaz and not with some other structure These questions are answered formally by the statement that in each case the structure with the minimum free energy is stable. This answer, however, is not satisfying what is desired in our atomistic and quantum theoretical era is the explanation of this minimum free energy in terms of atoms or ions and their properties. 

Fig. 2.—The arrangement of ions in cube-face layers of alkali halide crystals with the sodium chloride structure.

Figure 6.12 The structures of some typical color centers in alkali halide crystals (such as NaCl). The defects are represented on a plane of the alkali halide crystal. The circles represent the lattice ions and a is the anion-cation distance.

Rocket propulsion oxidizers, 18 384-385 Rocks, weathering of, radiation and, 3 299 Rocksalt, crystal structure of, 2 6, 29 Rock-salt-type alkali halide crystals, dissolution process, 39 411 19 alkali chlorides, 39 413, 416 alkali fluorides, 39 413-415... 

For a 1 1 solid MX, a Schottky defect consists of a pair of vacant sites, a cation vacancy, and an anion vacancy. This is presented in Figure 5.1 (a) for an alkali halide type structure the number of cation vacancies and anion vacancies have to be equal to preserve electrical neutrality. A Schottky defect for an MX2 type structure will consist of the vacancy caused by the ion together with two X anion vacancies, thereby balancing the electrical charges. Schottky defects are more common in 1 1 stoichiometry and examples of crystals that contain them include rock salt (NaCl), wurtzite (ZnS), and CsCl. 

It was Ziman [77] who has noted that there is little hope, at least at present, to develop an experimental technique permitting the direct measurement of these correlation functions. The only exception are the joint densities x / (r> ) information about which could be learned from the diffraction structural factors of inhomogeneous systems. On the other hand, optical spectroscopy allows estimation of concentrations of such aggregate defects in alkali halide crystals as Fn (n = 1,2,3,4) centres, i.e., n nearest anion vacancies trapped n electrons [80]. That is, we can find x mK m = 1 to 4, but at small r only. Along with the difficulties known in interpretating structure factors of binary equilibrium systems (gases or liquids), obvious specific complications arise for a system of recombining particles in condensed media which, in its turn, are characterized by their own structure factors. 

Fig. 16. Structures of F-, H-, and -centres in alkali halide crystals (+) is the ion of an alkaline metal, (—) is the halide ion.

Numerous data about the processes of the tunneling recombination of radiation defects have been obtained in studies on tunneling recombination luminescence. The recombination luminescence of y-irradiated alkali halide crystals was discovered in the mid-1960s [58, 59] in studying the transfer of electrons from Ag and T1 atoms (electron donors) to Cl2 particles (electron acceptor). The Ag and T1 atoms are formed as a result of the action of irradiation on alkali halide crystals which contain Ag+ or Tl+ additives in amounts of about 10 3M. The electrons generated by the irradiation reduce the Ag4 or TU ions to Ag° or Tl° while the hole centres are stabilized in the form of the Cl2 ion occupying two anion positions in the lattice. The hole centres of this kind, whose structure is depicted schematically in Fig. 16, are referred to as Vk-centres. 

Cesium chloride crystallizes with a structure derived from the simple cubic primitive cell. Ch ions occupy the 8 comer sites with Cs+ in the center of the cell note that this is not a body-centered cubic unit cell since the ion at the center is not the same as those at the comers. Thus there is one CsCl unit per unit cell and the coordination numbers of Cs+ and Ch are both 8. Crystals of CsBr and Csl adopt the CsCl structure, but all other alkali halides crystallize in the NaCl structure. 

An examination of the distances of shortest internuclear separation r0 between oppositely charged ions in alkali halide crystals with the sodium chloride structure led long ago to the concept of ionic (crystal) radii. For example, if the r0 values for the sodium and potassium salts of the same anion are inspected, it is found that there is an approximately constant difference, irrespective of the nature of the halide ... 

Figure 9-41. The arrangement of ions in cube-face layers of alkali halide crystals with the sodium chloride structure. Adaptation from Pauling [61], Copyright (1960) Cornell University. Used by permission of the publisher, Cornell University Press.

Cohen, A. J., and R. G. Gordon (1975). Theory of the lattice energy, equilibrium structure, elastic constants, and pressure-induced phase transitions in alkali-halide crystals. Phys. Rev. B12, 3228 1. 

Mobus, M., Karl, N. and Kobayashi, T. (1992). Structure of perylenetetracarboxylic dianhydride thin films on alkali halide crystal substances. /. Cryst. Growth, 116, 495-504. [263]... 

The first analyses of simple crystal structures were made using Laue photographs when W. L. Bragg deduced the structure of sodium chloride and other alkali halide crystals (for a historical review see Bragg (1975)). Subsequently, Bragg primarily used the monochromatic X-ray... 

The next phase for the theorists in connection with this work lies in predictions of helium atom scattering intensities associated with surface phonon creation and annihilation for each variety of vibrational motion. In trying to understand why certain vibrational modes in these similar materials appear so much more prominently in some salts than others, one is always led back to the guiding principle that the vibrational motion has to perturb the surface electronic structure so that the static atom-surface potential is modulated by the vibration. Although the polarizabilities of the ions may contribute far less to the overall binding energies of alkali halide crystals than the Coulombic forces do, they seem to play a critical role in the vibrational dynamics of these materials. 

Tasker, P.W. (1979) The surface energies, surface tensions and surfiice structure of the alkali halide crystals. Philos. Mag. A, 39 (2), 119-136. 

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. 

Rocksalt NaCl structure (see Fig. 2.6) has the symmetry of symmorphic space group N225 and is typical of alkali halide crystals (Li, Na, K fluorides, chlorides and bromides) and some oxides (MgO, CaO, SrO). 

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