Interstitial ions, solid electrolytes

The above two mechanisms may be regarded as isolated ion hops. Sometimes, especially in solid electrolytes, cooperative ion migration occurs. An example is shown in Fig. 2.1(c) for the so-called interstitialcy or knock-on mechanism. A Na" ion. A, in an interstitial site in the conduction plane of j -alumina (see later) cannot move unless it persuades one of the three surrounding Na ions, B, C or D, to move first. Ion A is shown moving in direction 1 and, at the same time, ion B hops out of its lattice site in either of the directions, 2 or 2. It is believed that interstitial Ag" ions in AgCl also migrate by an interstitialcy mechanism, rather than by a direct interstitial hop. 

As shown in Fig. 2.5, the cyclic voltammograms for Prussian blue attached to paraffin-impregnated graphite electrodes (PIGEs) in contact with aqueous electrolytes exhibit two well-defined one-electron couples. Prussian blue crystals possess a cubic structure, with carbon-coordinated Fe + ions and nitrogen-coordinated Fe + ions, in which potassium ions, and eventually some Fe + ions, are placed in the holes of the cubes as interstitial ions. The redox couple at more positive potentials can be described as a solid-state process involving the oxidation of Fe + ions. Charge conservation requires the parallel expulsion of K+ ions [77] ... 

Over a large range partial pressures of oxygen ionic conductivity dominates and the material behaves as a solid electrolyte. Under these conditions there is an equilibrium established between oxygen ion vacancies, interstitial oxygen ions and lattice oxygen. 

A gradient in electrostatic potential can produce a driving force for the mass diffusion of a species, as discussed in Section 2.2.2. Two examples of this are the potential-gradient-induced diffusional transport of charged ions in ionic conductors such as those used in solid-electrolyte batteries and the electron-current-induced diffusion of interstitial atoms in metals. 

Solid electrolyte — is a class of solid materials, where the predominant charge carriers are -> ions. This term is commonly used for -> conducting solids with ion -> transport number equal to or higher than 0.99 (see also -> electrolytic domain). Such a requirement can only be satisfied if the -> concentration and -> mobility of ionic -> charge carriers (usually -> vacancies or interstitials) both are relatively high, whilst the content of -> electronic defects is low. See also -> superionics, -> defects in solids, - diffusion, and -> Nernst-Einstein equation. 

In contrast with their oxide counterparts, CaFj and PbFj can form fluorite-type solid solutions with fluorides of higher-valency metals such as YFj, BiFj, UF4, etc. They are expressed, for example, by Cai xYxF2+x, in which the excess F" ions are accommodated in the interstitial sites, forming various kinds of clusters with Vp, depending on the concentration of F . They are usually good F solid electrolytes, and the conductivity of Pbj.xBixFj+x (x = 0.25) is shown in Figure 6.3 as an example. 

Rare earth oxy-apatites are also attracting considerable interest as a new type of solid electrolytes possessing a high oxide-ion conductivity at intermediate temperatures [22-35]. In contrast to traditional fluorite- and perovskite-type oxide electrolytes conducting via oxide ion vacancies, the high ion conductivity of the apatites is provided by the transport of interstitial oxide ions [29-34], This is caused by peculiarities of the apatite structure that tolerates different structural defects such as cation vacancies and oxygen interstitial sites. 

Diffusion and migration in solid crystalline electrolytes depend on the presence of defects in the crystal lattice (Fig. 2.16). Frenkel defects originate from some ions leaving the regular lattice positions and coming to interstitial positions. In this way empty sites (holes or vacancies) are formed, somewhat analogous to the holes appearing in the band theory of electronic conductors (see Section 2.4.1). 

Correlations of Nemst-Haskell [9] for electrolytes were mentioned. The effect of concentration, that is, dilnte versus concentrated solutions were separately discussed. Correlations of Wilkee-Chang, Siddiqi-Lucas, and Haydeek-Minhas were described. The diffusion mechanism in solids was discussed. The various mechanisms of diffusion such as vacancy mechanism, interstitial mechanism, snbstitu-tional mechanism, and crowd ion mechanism were outlined. Knudsen diffusion, when the mean free path of the molecule is greater than the diffusion path, as in pore diffusion, was discussed. Diffusion in polymers and the Arrhenius dependence of the diffusion coefficient with temperature were discussed. 

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