Anion interstitial vacancies

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

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

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. 

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. 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... 

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]

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. 

Nonequihbrium concentrations of point defects can be introduced by materials processing (e.g. rapid quenching or irradiation treatment), in which case they are classified as extrinsic. Extrinsic defects can also be introduced chemically. Often times, nonstoichiometry results from extrinsic point defects, and its extent may be measmed by the defect concentration. Many transition metal compounds are nonstoichiometric because the transition metal is present in more than one oxidation state. For example, some of the metal ions may be oxidized to a higher valence state. This requires either the introduction of cation vacancies or the creation of anion interstitials in order to maintain charge neutrality. The possibility for mixed-valency is not a prerequisite for nonstoichiometry, however. In the alkah hahdes, extra alkah metal atoms can diffuse into the lattice, giving (5 metal atoms ionize and force an equal number... 

Schottky defects occur when sites that are normally occupied by atoms or ions are left vacant. In order that the crystal structure maintain its electrical neutrality, for every cation-site vacancy there must be an anion-site vacancy. At room temperatures, one in 10 sites is typically vacant, but this adds up to 10 Schottky defects in a 1 mg crystal. A less commonly observed defect is a Frenkel defect, in which an atom or ion is displaced from its site to an interstitial site that is normally unoccupied. In so doing the number of nearest neighbors of one component of the crystal is changed. This type of defect is seen in... 

Figure 12.8 Binary ionic crystal showing defects that can lead to lattice diffusion, (a) Frenkel defect vacancy-interstitial pair), (b) Schottky defect (anion-cation vacancy). (After Kingery ct a .. 1976.)...

Presence of these interstices provides to the fluorite stmcture extremely specific features. In UO2 particularly, it allows for placement of some radioactive decay products, these sites are responsible for existence of hyperstoichiometric UO2+X phase, where the extra oxygen ions fill the empty interstitial sites in the fluorite lattice etc. First case is extremely important in radiation damaged UO2. Second one is cmcial in oxidation of pure UO2 in atmospheric conditions. Diffusion of atmospheric oxygen into the bulk of crystal brings excess oxygens into empty interstices. These become filled more or less randomly only at low x, at higher concentration of extra anions they form different types of clusters, including so-called 2 2 2 Willis dimers Willis), tetra- and pentameric defects clusters of cuboctahedral symmetry Allen and Tempest). Last defects appear due to interaction of extra anions with intrinsic crystal FP defects (anion Frenkel pairs, i.e. anion vacancies and anion interstitials). 

Figure 13. Schematic representation of some important diffusion species in ionic crystals, including cation and anion vacancies, cation and anion interstitials, cation and anion divacancies, cation-anion divacancy, allo-valent cations (e.g., REE). These are by no means all the likely defect species but illustrate the complex kinetics involved (modified after Borg and Dienes 1988).

Anti-Frenkel Disorder Equal concentrations of anion vacancies and anion interstitials... 

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