Crystals alkali halide properties

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. 

As we have seen, several atomic properties are important when considering the energies associated with crystal formation. Ionization potentials and heats of sublimation for the metals, electron affinities, and dissociation energies for the nonmetals, and heats of formation of alkali halides are shown in Tables 7.1 and 7.2. 

Overlapping Ion Model. The ground-state wave function for an individual electron in an ionic crystal has been discussed by Lowdin (24). To explain the macroscopic properties of the alkali halides, Lowdin has introduced the symmetrical orthogonaliz tion technique. He has shown that an atomic orbital, x//, in an alkali halide can be given by... 

F centers may act as adsorption centers not only in the alkali halides, but in any other crystals as well. Take, for example, a crystal of ZnO, in which the F center is an oxygen valency with two (not one ) electrons localized near it, as depicted in Fig. 30. From the chemical point of view such a center represents two adjacent localized free valencies of like sign which on an ideal surface could never meet because of Coulomb repulsion between them. (This should be especially stressed.) As a result of this property, such an F center may play a specific role in catalysis acting as an active center for a number of reactions. 

Binary compounds formed between metals and group 6 or group 7 elements usually occur in the form of ionic crystals rather than as isolated molecules. The most typical example is, of course, given by the alkali halides, studied by Lowdin in his classic treatise from 1948 [1], Another important class of ionic crystals, with somewhat different properties, are the metal oxides, which play a central role in many contexts in chemistry and physics. To mention only one example, their catalytic properties have long been recognized and subject to extensive study, and have given rise to numerous applications of enormous practical importance. 

Strongly for the ionic crystals, yet the bulk modulus for the alkali halides varies as d. The cl trend for the bulk modulus will show up in the study of simple metals, and in terms of the pseudopotentials that will be used in the study of simple metals, d" -dependence takes on a particularly fundamental role. In Problem 15-3, the simple metal theory is used to give a good account of the bulk modulus in C, Si, and Gc. It should be noted also that the simple metal theory docs not give a good account of cohesive energy itself there is much cancellation between terms for that property, and there are important contributions (for example, that do not vary as... 

The LCGTO-Xa approach described so far has been successfully applied to a large variety of systems, including main group molecules (50,52,53), transition metal compounds, e.g. carbonyl complexes (27,28,55,56) and ferrocene (57), and a number of transition metal dimers (47). Besides these investigations on ground state properties useful information has also been obtained for selected problems involving excited states (52), such as the photolysis of Ni(CO)4 (58,59) and localized excitons in alkali halides (60) and in other ionic crystals ( ). 

Fluorine is the most electronegative of all the elements, so that the bonds it forms with most other elements have considerable ionic character. With the exception of the alkali halides most crystalline fluorides have structures different from those of the other halides of the same metal. A number of difluorides and dioxides have the same crystal structure, whereas the corresponding dichlorides, dibromides, and diiodides have in many cases structures similar to those of disulphides, diselenides, and ditellurides. The extreme electronegativity of fluorine enables it to form much stronger hydrogen bonds than any other element, resulting in the abnormal properties of HF as compared with the other acids HX, the much... 

J.M. Peech, D.A. Bower and R.O. Rohl, Preparation of Pure Alkali Halide Crystals and Some of Their Properties, J. Appl. Phys. 38 (1967) 2166-2171. 

Early applications of pseudopotentials in cluster models [62,63], which dealt with impurities in alkali halide crystals, used Hartree-Fock (HF) based model potentials [64] and complete-cation norm-conserving pseudopotentials [65]. A similar technique was found valuable to describe bulk properties of alkaline-earth oxides [66-68]. A general procedure for calculating embedded clusters under the assumption of a frozen environment and orthogonality requirements for the wave function of the cluster and the environment was also discussed... 

Three metal oxides with fee lattices and rocksalt stmetures have been examined by HAS NiO [78, 79], CoO [51, 80], and MgO [81-83], Because the ions in these materials are divalent, the atomic interaction forces are very much stronger than those for the alkali halides. This results in substantial differences in a number of physical properties, including the very high melting and boiling points of the oxides and the hardness of their crystals. Nonetheless, they still cleave along (100) planes like the alkali halides to form high-quality surfaces. 

Two of the alkali halides have been used as a scintillator material, viz. Nal and Csl, both doped with Tl. Table 9.5 summarizes some of their properties. Also included are Csl Na and undoped Csl. The emission spectra of the TI+-doped crystals ate given in Fig. 9.9. 

The presence of defects has been found to influence the bulk as well as surface characteristics of alkali halides. Basically, these defects alter the charge neutrality on the cleavage planes, leading to a change in the surface properties of uni-univalent alkali halides. In this regard, oxygen defects have been either introduced into or removed from the crystal lattice in order to examine their influence on the surface properties of KCl and NaCl. The KCl and NaCl salts were selected for these studies because they have been shown to be very stable hosts for various types of defects and also represent the most important alkali halide salts that deviate from the simplified lattice ion hydration theory. 

The F-center is the most fundamental color center defect in the alkali halide lattice. Although it is not laser-active, the optical properties of the F-center are important in understanding the laser physics of other color center lasers. The fundamental absorption band of the F-center, called the F band, corresponds to a transition fi om the Is-like ground state to the 2p-like first excited state of the square-well potential. The F-band transition is very strong, and dominates the optical spectrum of the alkali-halide crystal. In fact, the term F-center comes fi om the German word Farbe, meaning color, and refers to the strong color imparted to the otherwise transparent alkali-halide crystals. 

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