Alkali halides effective field

Except for the alkali halides, however, there have been few attempts at investigating this effect theoretically. Nevertheless, it is usually small except when the field gradients themselves are small (the reasons for this will become apparent shortly) and in most cases field gradients have been estimated theoretically only for the molecule in its stationary equilibrium configuration. 

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... 

Let us say a few words on non-selective techniques which have been employed in the study of Li+ systems [93,152]. If this impurity moves off-centre in a alkali halide lattice the pair formed by the positive vacancy and the Li+ ion gives rise to an electric dipole, p. It is well known that free dipoles under an applied electric field, E, tend to place p parallel to E while thermal disorder is opposed to this effect. For this reason, if the average value of p at a given temperature is designated by... 

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. 

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). 

The partial molar volume is a thermodynamic quantity that plays an essential role in the analysis of pressure effects on chemical reactions, reaction rate as well as chemical equilibrium in solution. In the field of biophysics, the pressure-induced denaturation of protein molecules has continuously been investigated since an egg white gel was observed under the pressure of 7000 atmospheres [60]. The partial molar volume is a key quantity in analyzing such pressure effects on protein conformations When the pressure in increased, a change of the protein conformation is promoted in the direction that the partial molar volume reduces. A considerable amount of experimental work has been devoted to measuring the partial molar volume of a variety of solutes in many different solvents. However, analysis and interpretation of the experimental data are in many cases based on drastically simplified models of solution or on speculations without physical ground, even for the simplest solutes such as alkali-halide ions in aqueous solution. Matters become more serious when protein molecules featuring complicated conformations are considered. 

Lyotropic numbers N o, Table 5.1, were assigned to ions in the 30s of the last century by Buchner and Voet (Buchner et al. 1932 Voet 1937a, 1937b) according to their effects on colloidal systems. The lyotropic series has nowadays been to some extent superseded by the Hofmeister series, with which it is taken to be practically synonymous, but it is not so exactly. For the alkali metal cations and the halide anions the lyotropic numbers obtained from colloidal phenomena are linearly related to their enthalpies of hydration. Voet (1937a) concluded that the lyotropic series are simply related to the electric field strengths of the ions. Note that the Myo values for the alkali metal cations are not commensurate with those of the alkaline earth cations and with those of the anions. 

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