Anion interstitial

Anion Interstitials The other mechanism by which a cation of higher charge may substitute for one of lower charge creates interstitial anions. This mechanism appears to be favored by the fluorite structure in certain cases. For example, calcium fluoride can dissolve small amounts of yttrium fluoride. The total number of cations remains constant with Ca +, ions disordered over the calcium sites. To retain electroneutrality, fluoride interstitials are created to give the solid solution formula... 

In the oxidized region, holes and anion interstitials are preferred, so the electroneutrality equation is approximated by... 

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

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. 

Generally, anion interstitials are rare, because the anionic radius is greater than the cationic radius. The rule of electrical neutrality in a material containing both Schottky and Frenkel defects requires that the positive and negative point defects must be balanced, that is... 

Fig. 7. Diagram of Gibbs free energy AG for the ion transfer through a passive layer with a Free Activation energy AG between two favoured sites for anions (interstitials or vacancies) under the influence of a high electrical field strength and without field (horizontal curve) explaining the high field mechanism for oxide growth.

Fig. 2. Field-controlled oxide growth, (a) Surface and interfacial charges (b) Cation interstitial (ci) and anion interstitial (ai) currents.

Fig. 4. Bulk ionic defect concentrations at the oxide interfaces associated with equilibrium interfacial reactions. [At the metal—oxide interface, there are cation interstitital (ci) and anion vacancy (av) species, while at the oxide—oxygen interface, there are anion interstitial (ai) and cation vacancy (cv) species],...

Note in Fig. 4 that there are four primary ionic defect species which we must consider, namely cation interstitials, cation vacancies, anion interstitials, and anion vacancies (denoted by the superscripts ci, cv, ai and av, respectively). In the case of non-simultaneous place exchange, referred to... 

Similarly, a flow of anion interstitials from the oxide—oxygen interface to the metal—oxide interface can lead to new oxide growth. We can write... 

Fig. 8. Schematic diagrams of concentration profiles and the associated particle currents, (a) Cation interstitials or anion vacancies [(dC/dx)<0] and positively directed particle currents ( 0). (b) Cation vacancies or anion interstitials [(dC/dx) > 0] and negatively directed particle currents (J< 0).

Fig. 9. Electric field polarities produced by the easy diffusion of various charged defect species, (a) Cation interstitials (b) anion interstitials (c) cation vacancies ...

Fig. 14. Possible sign combinations involving the sign of the interfacial charge at the oxide—oxygen interface and the sign of the charge of the field-driven mobile species originating at the oxide—oxygen interface, together with schematic diagrams of the concentraion profiles for the mobile species, (a) Field-driven positive-hole transport (b) field-driven anion interstitial (or cation vacancy) transport.

Since local space-charge neutrality does not hold at the oxide interfaces, the above expression for the current is restricted to the interior zone [28] where local space charge neutrality has been found [46] to be a good approximation. This is illustrated for the case of cation vacancy (or anion interstitial) and electron-hole diffusion by Fig. 17. Thus, the domain of validity is not 0 but instead is 5 < [Pg.75]

Figure 21 indicates the anion interstitial currents through the layers. The coordinate system is chosen to be the same as that already utilized for cation interstitial diffusion, namely that illustrated in Fig. 18. Because all anion interstitials originate at the oxygen interface, the anion interstitial currents decrease in magnitude in the order i + 1, i, i — 1,. . . , as noted in Fig. 21. The difference current Jf(ai) — J VI serves to increase the thickness...

Fig. 21. One of the interior oxide layers labeled i in a sandwich array of multilayered oxides growing by anion interstitial (ai) diffusion illustrated with the relative magnitudes of the negatively directed particle currents through layer i and the adjacent layers i — 1 and i + 1.

The c/ vi is thus not an oxygen anion interstitial current, but is introduced merely to cast the growth equation for layer N into a form analogous to that for the inner layer growth. The N is determined by eqn. (229). 

To summarize the case of anion interstitial diffusion, the growth equations for the N layers can be written in the form... 

It is very informative to compare the set of equations for the anion vacancy case with the corresponding set for anion interstitial diffusion. There is a one-to-one correspondence, with the slight differences being due to the appearance of 2 f(av) in place of in the coefficients. This can be noted most easily by referring to Table 1. Our conclusion is that the growth equations based on anion diffusion are essentially independent of whether the anion diffusion occurs by an interstitial or a vacancy mechanism. 

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