Alkali metal halides crystals

Van Geet has used a similar approach to estimate the chemical shifts of solvated ions (15). He argues that if the repulsive overlap mechanism, shown by Richards et al. (16) to work for ion-ion interactions, also works for ion-solvent interactions, then the repulsive overlap mechanism used by Kondo and Yamashita (17) for alkali metal halide crystals justifies the above assumptions. 

Self-Luminescence. The action of UV light or ionizing radiation on pure alkali-metal halide crystals causes intense luminescence particularly at low temperature. The emission spectrum is characteristic for each individual compound. This fluorescence is comparable with the recombination luminescence which occurs upon capture of an electron by a VK center (defect electron). 

Expressions for the force constant, i.r. absorption frequency, Debye temperature, cohesive energy, and atomization energy of alkali-metal halide crystals have been obtained. Gaussian and modified Gaussian interatomic functions were used as a basis the potential parameters were evaluated, using molecular force constants and interatomic distances. A linear dependence between spectroscopically determined values of crystal ionicity and crystal parameters (e.g. interatomic distances, atomic vibrations) has been observed. Such a correlation permits quantitative prediction of coefficients of thermal expansion and amplitude of thermal vibrations of the atoms. The temperature dependence (295—773 K) of the atomic vibrations for NaF, NaCl, KCl, and KBr has been determined, and molecular dynamics calculations have been performed on Lil and NaCl. Empirical values for free ion polarizabilities of alkali-metal, alkaline-earth-metal, and halide ions have been obtained from static crystal polarizabilities the results for the cations are in agreement with recent experimental and theoretical work. 

The adsorption of H2O on a variety of alkali-metal halide crystals has been studied by i.r. and far-i.r. spectroscopy. For the majority of halides (typically... 

In solid-state inorganic chemistry. MCD is an important technique for the determination of color centers in alkali-metal halide crystals, rare earths, and highly symmetric compounds such as hexaha-lides of platinum or ruthenium. In organic chemistry, MCD is observed with, e,g.. metal poiphy-rins, pyrimidine ba.ses, and nucleoside derivatives. 

The alkali metal halides are all high-melting, colourless crystalline solids which can be conveniently prepared by reaction of the appropriate hydroxide (MOH) or carbonate (M2CO3) with aqueous hydrohalic acid (HX), followed by recryslallization. Vast quantities of NaCl and KCl are available in nature and can be purihed if necessary by simple crystallization. The hydrides have already been discussed (p. 65). 

Ionic bond, 287, 288 dipole of, 288 in alkali metal halides, 95 vs. covalent, 287 Ionic character, 287 Ionic crystal, 81, 311 Ionic radius, 355 Ionic solids, 79, 81, 311 electrical conductivity, 80 properties of, 312 solubility in water, 79 stability of, 311... 

Ionic Cations and anions Electrostatic, non-directional Hard, brittle, crystals of high m.t. moderate insulators melts are conducting Alkali metal halides... 

Alkali-Metal Halides. Luminescent alkali-metal halides can be produced easily in high-purity and as large single crystals. They are therefore often used as model substances for the investigation of luminescence processes. Their luminescence processes can be divided into 1) the self-luminescence of the undoped crystals, 2) luminescence by lattice defects, and 3) sensitized luminescence. 

Salts of [ZnLe]2+ (L = pyridine N-oxide) have recently been shown to undergo facile solid state reactions with alkali metal halides, an observation to be taken into account when recording the IR spectra of these and related compounds.706 A crystal structure of the complex [ZnL6] [C104]2 (L = 4-methylpyridine A-oxide) has been reported.707 The metal is in a near-octahedral 06 environment, with an average Zn—O distance of 2.114 A. 

Other cases of approximately monatomic chromophores occur in 4f-+5d transitions now known in Sm11, Eu11, Tm11,28 Ybn, Cera, Prm, and Tbra.16 (The half-filled shell effect expressed by Eq. (3) is very conspicuous in this distribution of known species.) 5transitions are known in UIU, Np111, Puin, Paiy, U, Np, and Pu17. 5s-+5p transitions are known in complexes of Snn and Sbm and 6s — 6p in Tl1, Pbn, and Bira. The halide ions in solvents of not too high electron affinity and in crystals of alkali metal halides show absorption bands which to a certain approximation can be described as 3p - 4s(Cl), 4p — -5s(Br), and 5p - -6s(I). 

A set of empirical ionic radii can be derived from the direct measurement of internuclear distances in crystal structures. The additivity of ionic radii is substantiated by the near constancy of the differences in internuclear distances Ar between the alkali metal halides, as shown in Table 4.2.1. 

A major difference between crystals and fluids refers to the necessity of distinguishing between different sites. So the autoprotolysis in water could, just from a mass balance point of view, also be considered e.g. as a formation of a OH vacancy and a IT vacancy. In solids such a disorder is called Schottky disorder (S) and has to be well discerned from the Frenkel disorder (F). In the densely packed alkali metal halides in which the cations are not as polarizable as the Ag+, the formation of interstitial defects requires an unrealistically high energy and the dominating disorder is thus the Schottky reaction... 

Bragg (1912) showed a sodium chloride crystal to consist, not of discrete molecules of NaCl, but of Na+ ions and Cl ions arranged in an indefinitely extended cubic lattice (Fig. 28, exterior view Fig. 84 on p. 138 shows co-ordination) X-ray analysis (p. 141) gave the internuclear distance as 2.81 A. Most alkali metal halides have the same type of crystal lattice but the inter-ionic distances differ. 

All the alkali metal halides except the cliloride, bromide and iodide of caesium form cubic crystals with the rock salt lattice and show a co-ordination number of 6. The exceptions are also cubic, but have the caesium chloride structure (Fig. 133) characterised by a co-ordination number of 8. The radius ratio for CsCl, Cs /Cl" = 0.93, allows 8 co-ordination, but is so near the ratio for 6 co-ordination that caesium chloride is dimorphous, changing, at 445°, from the caesium chloride to the rock salt structure. The crystalline halides are generally markedly ionic, though, as expected, lithium iodide is somewhat covalent, for iodide is the largest and most easily polarised simple anion and lithium, the smallest alkali metal cation, possesses the strongest polarising power. 

Alkali metal halide melts are characterized by a quasi-crystalline structure originating by dilatation of the crystal structure and by the occurrence of different kinds of positional disordering. Cations and anions are preferably surrounded by ions of the opposite charge... 

The physical state of a sample and the information required has to be considered when deciding how best to carry out an infrared analysis of a sample. As has been mentioned previously, sample preparation can be very important and there are examples where this is true for colorants analysed by infrared. For example, if the polymorphism (capable of existing in more than one crystal form) of a colorant is to be studied, then the sample preparation step(s) should not physically nor chemically alter the sample, that is, minimal and mild sample preparation should be used (ruling out the use of the alkali metal halide disk technique, where grinding can cause conversion in crystal forms). 

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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