Migration interstitial anion

A similar relation holds for anionic vacancies and for diffusion through interstitial migration (replace [Vm] and TTym. nd so on). 

Vm = migration enthalpy of cationic vacancy = migration enthalpy of anionic vacancy /7m = migration enthalpy of cationic interstitial //xj = migration enthalpy of anionic interstitial... 

If the semiconductor is an ionic solid, then electrical conduction can be electronic and ionic, the latter being due to the existence of defects within the crystal that can undergo movement, especially Frenkel defects (an ion vacancy balanced by an interstitial ion of the same type) and Schottky defects (cation and anion vacancies with ion migration to the surface). This will be discussed further in Chapter 13, as ionic crystals are the sensing components of an important class of ion selective electrodes. 

In the former case, the ions migrate among the interstitial defects, which may be relevant only to small ions such as Li+. This leads to a transference number close to 1 for the cation migration. In the other case, the lattice contains both anionic and cationic holes, and the ions migrate from hole to hole [39], The dominant type of defects in a lattice depends, of course, on its chemical structure as well as its formation pattern [40-43], In any event, it is possible that both types of holes exist simultaneously and contribute to conductance. It should be emphasized that this description is relevant to single crystals. Surface films formed on active metals are much more complicated and may be of a mosaic and multilayer structure. Hence, ion transport along the grain boundaries between different phases in the surface films may also contribute to conductance in these systems. 

For strongly ionic solids where cation and anion sizes are comparable, e.g. NaCf, KBr, etc., Schottky defects will predominate and both transport numbers and are greater than zero t = 0, no current is carried by electron migration). When the size of the cation is considerably smaller than the anion, eg. AgBr, AgCf, etc., Frenkel defects occur and the interstitial cations are the dominant current carriers 1). 

Anion migration could also occur by the first mechanism, but for the second, it is usually the cation that is small enough to occupy an interstitial site, for example, the tetrahedral holes in an NaCl-type structure. 

Belous et al. [238] investigated the role of impurities as they affect luminescence and photolysis of silver azide. Subsequent to X-ray irradiation, luminescence appeared on UV excitation and was associated with the same interstitial Ag ions which influence the photolysis kinetics. By studying the growth of luminescence, it was concluded that the migration of energy to centers where decomposition occurs is via excitons, presumably excited anion states. 

When Schottky defects are present in a crystal, vacancies are found on the cation and the anion sublattices, allowing both cation and anion diffusion to occur (Figure 7.11a). In the case of Frenkel defects, interstitial sites are occuppied and vacancies occur, allowing for interstitial, interstitialcy and vacancy diffusion to take place in the same crystal (Figure 7.11b) so that three migration routes are possible. 

Frenkel defects. The case where only the cation is mobile can be explained by assuming that the anion lattice is perfect but that the cation lattice contains cation vacancies and interstitials in equivalent concentrations to maintain electroneutrality for the wholecrystal. This type of defect is found in the silver halides and is shown for AgBr in Figure 3.2. The cation in this case is free to migrate over both vacancy and interstitial sites. 

When metals react with gases, the main corrosion products are ionic compounds that can be stoichiometric or nonstoichiometric. Generally, only defect ions (ion condnctors) arise in stoichiometric componnds snch as silver chloride (AgCl) and NaCl. Four border cases of imperfections are possible When cation vacancies in the lattice and cations at interstitial lattice sites are found in an undisturbed anion lattice, the cations are mobile. Alternatively, the anions are mobile. In compounds with anion and cation vacancies, both can migrate, as they can when an equal number of cations and anions are present at interstitial lattice sites. 

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