Alkali halides effective charges

Hanlon JE, Lawson AW (1959) Effective ionic charge in alkali halides. Phys Rev 113(2) 472-478... 

If both L ro and l ro, the Arrhenius law takes place with the effective activation energy E equal to that of the Coulomb attraction just after the creation of charged pair, similar to the case of elastic interaction. Note that since the actual value of the Onsager radius L in alkali halides could be as large as L 100 A (T = 100 K, e 5, — —ee = e), only a small... 

Recalculate the effective ionic charge Z and transvcr.se charge ef for alkali halides in the CsC l structure, using the tjg from Table 14-2. 

In Section 19-E we found contributions to the effective charge 7 of the oxygen due to the matrix clement between an sp hybrid and a d slate and due to the matrix element For an oxygen displacement perpendicular to the Ti—O axis, the predicted transverse charge is simply this Z. However, for displacements along the Ti—O axis there are contributions from the transfer of charge between ions just as there were in the alkali halides, for which we obtained the transverse charge in Section 14-C. 

The simplest examples of this type of defect are provided by the alkali halides, studied extensively by Pohl and his colleagues (2). Sodium chloride heated in sodium vapour assimilates sodium atoms, which occupy normal cation sites as TSIa+ ions, while the extra electrons are trapped in the neighbourhood of the newly-created vacant anion sites. Vacant anion sites act as centres of effective positive charge in the crystal and produce a Coulomb-like potential field capable of binding electrons. This will be discussed in more detail in 2.2. It is difficult... 

The effect of polarization is evident in the gaseous alkali-halide molecules, being most pronounced for small and large X (Lil) and large M and small X (CsF), and resulting in a dipole moment appreciably less than the product of the internuclear distance d) and the electronic charge (e) ... 

Solvent and salt effects on the equilibrium constants with V are very pronounced for negatively charged substrates (35, 40), which is of relevance for catalytic investigations. Thus, addition of buffer salts or alkali halides as nucleophiles at concentrations as low as 0.01 M decreases the complexation significantly. An opposite salting out effect on purely lipophilic substrates, for example, hydrocarbons, can only be expected at salt concentrations of 1 M. If lipophilic solvents, such as methanol, are added, for example, to circumvent solubility problems, the association constants are again lowered to a degree that may prohibit catalytic applications. 

Perhaps the most striking feature of a preliminary consideration of the electronic states of lead azide is the diversity of types of excitonic states that it may have. In addition to the charge-transfer excitons, well known in alkali halides, and effective mass excitons, well known in elemental semiconductors, one predicts for PhN intra-cation excitons (describable in terms of excited states of Pb +, and states of the Is 5d %s6p configuration, modified by the complex crystal field of the PbNg structure) and intra-anion excitons (describable in terms of excited states of Nj"). These offer possibilities for the transport of electronic energy. 

Halides.—Theoretical calculations have been performed on both alkali-metal halide molecules and crystals. " In an analysis of the dipole moments of alkali-metal halide molecules, the extent of effective charge transfer was found to vary from 0.76 (Lil) to 0.99 (CsF), in an order that is predictable from the Periodic Table. 

Diffusion in ionic materials occurs primarily by the movement of charged species. Therefore, the application of an electric field can provide a very powerful driving force for mass transport. There have been numerous studies on the effects of electric fields on transport phenomena. Several studies have been performed on the evaporation of alkali halides in the presence of an external field. These investigations showed that the application of an electric field enhanced the evaporation of the crystal species. Similar studies have been performed on oxide ionic conductors, including ZrOi and p-aluminas. However, only a few experiments have been performed on classical insulating oxides such as a-A Os and MgO (perhaps because they are insulators). 

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