Halides, alkali impurities

The appearance of the alkali alkoxide can lead to formation of bimetallic alkoxides or bimetallic alkoxide halides as impurities. For example, lithium oxoalkoxocomplexes were isolated as side products of the anodic dissolution of Mo and W when LiCl was used as conductive additive [908],... 

FIGURE 1 A representative sample of color centers in alkali halide crystals. The large and small circles represent negative and positive ions, respectively. Colored circles represent alkali impurities. 

Several authors of papers published in the 1960s and 1970s came to the conclusion that divalent impurities, includ g R ions, form in alkali-halide hosts impurity-vacancy (I-V) dipoles. Kao and Perlman (1979) have also accepted the 1-V dipole model for KCl Eu. They studied the relaxation and aggregation processes in these dipoles. It has been observed that X-irradiation destroys the dipoles and reduces the Eu " to Eu. References to earlier papers dealing with the I-V dipole model are cited in the above paper. 

Finally, moderate amounts of alkali impurities have been intentionally introduced into silica scales growing on both SiC and Si3N4 by vapor techniques (Pareek and Shores, 1991 McNallan et al., 1994 Sun et al., 1994). Alkali halides or alkali salts are vaporized in one portion of a furnace and transported with a carrier gas to the test specimen in another zone of the furnace. Depending on the activity of the alkali vapor species in the test, the oxide scale composition varied from 0.4 (Pareek and Shores, 1991) to 30 (Sun et al., 1994) mole percent alkaline oxide. Oxidation kinetics for the lowest levels of alkali impurity in the scale were parabolic, but elevated over rates ob-... 

Meng J, Pandey R, Vail J M and Kunz A B 1989 Impurity potentials derived from embedded quantum olusters Ag" and Cu" transport In alkali halides J. Phys. Condens Matter 1 6049-58... 

Materials that contain defects and impurities can exhibit some of the most scientifically interesting and economically important phenomena known. The nature of disorder in solids is a vast subject and so our discussion will necessarily be limited. The smallest degree of disorder that can be introduced into a perfect crystal is a point defect. Three common types of point defect are vacancies, interstitials and substitutionals. Vacancies form when an atom is missing from its expected lattice site. A common example is the Schottky defect, which is typically formed when one cation and one anion are removed from fhe bulk and placed on the surface. Schottky defects are common in the alkali halides. Interstitials are due to the presence of an atom in a location that is usually unoccupied. A... 

The common impurities found in amines are nitro compounds (if prepared by reduction), the corresponding halides (if prepared from them) and the corresponding carbamate salts. Amines are dissolved in aqueous acid, the pH of the solution being at least three units below the pKg value of the base to ensure almost complete formation of the cation. They are extracted with diethyl ether to remove neutral impurities and to decompose the carbamate salts. The solution is then made strongly alkaline and the amines that separate are extracted into a suitable solvent (ether or toluene) or steam distilled. The latter process removes coloured impurities. Note that chloroform cannot be used as a solvent for primary amines because, in the presence of alkali, poisonous carbylamines (isocyanides) are formed. However, chloroform is a useful solvent for the extraction of heterocyclic bases. In this case it has the added advantage that while the extract is being freed from the chloroform most of the moisture is removed with the solvent. 

Many inorganic solids lend themselves to study by PL, to probe their intrinsic properties and to look at impurities and defects. Such materials include alkali-halides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. 

Apart from halide and protic impurities, ionic liquids can also be contaminated with other ionic impurities from the metathesis reaction. This is especially likely if the alkali salt used in the metathesis reaction shows significant solubility in the... 

In some ionic crystals (primarily in halides of the alkali metals), there are vacancies in both the cationic and anionic positions (called Schottky defects—see Fig. 2.16). During transport, the ions (mostly of one sort) are shifted from a stable position to a neighbouring hole. The Schottky mechanism characterizes transport in important solid electrolytes such as Nernst mass (Zr02 doped with Y203 or with CaO). Thus, in the presence of 10 mol.% CaO, 5 per cent of the oxygen atoms in the lattice are replaced by vacancies. The presence of impurities also leads to the formation of Schottky defects. Most substances contain Frenkel and Schottky defects simultaneously, both influencing ion transport. 

Figure 9.5 Comparison of theoretical and measured hardnesses of pure alkali halides. The sohd circles are values at zero impurities extrapolated from measured values at known compositions.

As mentioned above alkali halide crystals are strongly hardened by small additions of divalent impurities. Data are available for 12 cases, the host crystals NaCl, NaBr, KC1, and KBr with additions of Ca2+, Sr2+, and Ba2+ (Chin, et al., 1973). It was found that the hardness increases depend only on the concentrations of the additions and not on the divalent specie (Ca, Sr, or Ba). However, a dependence on the valence (1, 2, or 3) is observed. It was also found that hardness increment is proportional to the square root of the concentration, (C1/2). 

Excess reductant and alkali halide are evaporated from the reaction mixture leaving, however, non volatile impurities in the metal, including oxygen from incompletely dehydrated starting halides. 

Dipolar ions like CN and OH can be incorporated into solids like NaCl and KCl. Several small dopant ions like Cu and Li ions get stabilized in off-centre positions (slightly away from the lattice positions) in host lattices like KCl, giving rise to dipoles. These dipoles, which are present in the field of the crystal potential, are both polarizable and orientable in an external field, hence the name paraelectric impurities. Molecular ions like SJ, SeJ, Nf and O J can also be incorporated into alkali halides. Their optical spectra and relaxation behaviour are of diagnostic value in studying the host lattices. These impurities are characterized by an electric dipole vector and an elastic dipole tensor. The dipole moments and the orientation direction of a variety of paraelectric impurities have been studied in recent years. The reorientation movements may be classical or involve quantum-mechanical tunnelling. 

Ions like S2, SeJ and SSe are found to align along the <110> directions in most alkali halides, while in Nal, KBr and KI, the S—bond of S2 is oriented along the <100> direction. In the case of Oj, the p orbitals are parallel to the <100> direction in sodium salts but are parallel to the direction in rubidium and potassium salts. Extensive spectroscopic studies have been reported on molecular ions such as NO, NO3, Cr04 and MnO . Several reviews(Corish et al, 1977 Bridges, 1975 Grimes, 1976) are available on such impurity-doped solids. 

The majority of inorganic systems reported to exhibit photochromism are solids, examples being alkali and alkaline earth halides and oxides, titanates, mercuric chloride and silver halides.184 185 The coloration is generally believed to result from the trapping of electrons or holes by crystal lattice defects. Alternatively, if the sample crystal is doped with an impurity capable of existing in variable oxidation states (i.e. iron or molybdenum), an electron transfer mechanism is possible. 

Particle irradiation effects in halides and especially in alkali halides have been intensively studied. One reason is that salt mines can be used to store radioactive waste. Alkali halides in thermal equilibrium are Schottky-type disordered materials. Defects in NaCl which form under electron bombardment at low temperature are neutral anion vacancies (Vx) and a corresponding number of anion interstitials (Xf). Even at liquid nitrogen temperature, these primary radiation defects are still somewhat mobile. Thus, they can either recombine (Xf+Vx = Xx) or form clusters. First, clusters will form according to /i-Xf = X j. Also, Xf and Xf j may be trapped at impurities. Later, vacancies will cluster as well. If X is trapped by a vacancy pair [VA Vx] (which is, in other words, an empty site of a lattice molecule, i.e., the smallest possible pore ) we have the smallest possible halogen molecule bubble . Further clustering of these defects may lead to dislocation loops. In contrast, aggregates of only anion vacancies are equivalent to small metal colloid particles. 

Fig. 3.4. Processes defending the survival probability of F centres in alkali halide crystals 1 -tunnelling recombination of close F, H defects, 2 - their annihilation, 3 - trapping of mobile H centre at impurity, 4 - formation of immobile dimer centre, 5 - H-centre leaves its geminate partner in random walks on a lattice.

The presence of lattice defects and/or intentionally placed impurities in the alkali halide crystal will cause the formation of local energy levels in the forbidden gap, called traps or activator centers. Figure 18.19 shows the energy levels of an alkali halide crystal, including the activator centers and traps. (Atomic thallium is a common activator for alkali halide crystals.)... 

The pure diol may be distilled unchanged, but traces of alkali or alkaline earth hydroxides or halides may cause explosive decomposition during distillation. In presence of strong acids, mercury salts may cause violent decomposition of the diol. See other ACETYLENIC COMPOUNDS, CATALYTIC IMPURITY INCIDENTS... 

In simple ionic crystals, such as the alkali halides, especially those containing ionic impurities, the process often stops at the photoionization stage, but in most molecular crystals, and even molecular ionic... 

The triboluminescence of minerals has been studied visually (see the footnotes to Table I) but only a few minerals have been examined spectroscopically. There are a few clear examples of noncentric crystals, such as quartz, whose emission is lightning, sometimes with black body radiation. Most of the triboluminescent minerals appear to have activity and color which is dependent on impurities, as is the case for kunzite, fluorite, sphalerite and probably the alkali halides. Table I attempts to distinguish between fracto-luminescence and deformation luminescence, but the distinctions are not clear cut. A detailed analysis of the structural features of triboluminescent and nontriboluminescent minerals may make it possible to draw conclusions about the nature and concentration of trace impurities that are not obvious from the color or geological site of the crystals. Triboluminescence could be used as an additional method for characterizing minerals in the field, using only the standard rock hammer, with the sensitive human eye as a detector. 

The work of Yamamoto (43) with growth-active impurities such as Pb+2, Sn+2, and Mn+2 ions reveals that their presence in very small quantities decreases the probability of nucleation, thus extending the metastable region of aqueous solutions of the alkali halides. These ions of the transition elements because of their screening demands withdraw Cl- ions from solution and form complexes such as (PbCl6) 4 and (MnCl6)-4. Without changing the over-all composition of the solution, these ions lower the effective concentration of the Cl ions and thus decrease the nucleation rate of NaCl. 

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