Defect concentrations, oxides

For all /7-type oxides, the defect concentration, and hence the electrical conductivity, increases with the oxygen pressure. 

Each breakdown is accompanied by some sound effect and is followed by a steady degradation of properties.284 It can also lead to a complete destruction of the oxide with visible fissures and cracks.286 The particular behavior observed depends on a large number of factors (electrolyte concentration,287 defect concentration in the oxide,288 etc.). The breakdown of thin-film systems (M-O-M and M-O-S structures) as a rule leads to irreversible damage of oxide dielectric properties.289... 

The close connection between ionic conductivity and diffusion means that the role of defect concentrations will be similar to that discussed in the previous chapter (Sections 5.8 and 5.9). For crystals such as oxides and halides with close-packed... 

From the formation reaction of protonic defects in oxides (eq 23), it is evident that protonic defects coexist with oxide ion vacancies, where the ratio of their concentrations is dependent on temperature and water partial pressure. The formation of protonic defects actually requires the uptake of water from the environment and the transport of water within the oxide lattice. Of course, water does not diffuse as such, but rather, as a result of the ambipolar diffusion of protonic defects (OH and oxide ion vacancies (V ). Assuming ideal behavior of the involved defects (an activity coefficient of unity) the chemical (Tick s) diffusion coefficient of water is... 

We thus ask What causes CS planes to nucleate (i.e. what are the reasons for anion vacancy aggregation and collapse in an oxide catalyst) and grow. We examine the response of defects in oxidizing atmospheres and, in particular, the role of anion vacancy concentrations in catalytic oxides. The EM results have led to novel concepts in oxidation catalysis (Gai 1981, 1992-1993, Gai et al 1982). 

Finally, the self-consistency of this general scheme is well supported by the equivalence between defect concentrations of impurity-containing nickel oxide as determined chemically and electrically (49,55). 

If majority point defect concentrations depend on the activities (chemical potentials) of the components, extrinsic disorder prevails. Since the components k are necessarily involved in the defect formation reactions, nonstoichiometry is the result. In crystals with electrically charged regular SE, compensating electronic defects are produced (or annihilated). As an example, consider the equilibrium between oxygen and appropriate SE s of the transition metal oxide CoO. Since all possible kinds of point defects exist in equilibrium, we may choose any convenient reaction between the component oxygen and the appropriate SE s of CoO (e.g., Eqn. (2.64))... 

In other cases, however, and in particular when sublattices are occupied by rather immobile components, the point defect concentrations may not be in local equilibrium during transport and reaction. For example, in ternary oxide solutions, component transport (at high temperatures) occurs almost exclusively in the cation sublattices. It is mediated by the predominant point defects, which are cation vacancies. The nearly perfect oxygen sublattice, by contrast, serves as a rigid matrix. These oxides can thus be regarded as models for closed or partially closed systems. These characteristic features make an AO-BO (or rather A, O-B, a 0) interdiffusion experiment a critical test for possible deviations from local point defect equilibrium. We therefore develop the concept and quantitative analysis using this inhomogeneous model solid solution. 

Crystal Self-Diffusion in Nonstoichiometric Materials. Nonstoichiometry of semiconductor oxides can be induced by the material s environment. For example, materials such as FeO (illustrated in Fig. 8.14), NiO, and CoO can be made metal-deficient (or O-rich) in oxidizing environments and Ti02 and Zr02 can be made O-deficient under reducing conditions. These induced stoichiometric variations cause large changes in point-defect concentrations and therefore affect diffusivities and electrical conductivities. 

In scheelite-type systems containing divalent ions, the tolerance for vacancies was more limited than in systems containing substantial amounts of monovalent ions. A vacancy limit of about 7.5% appeared to prevail at calcination temperatures between 550° and 800°C (97). The results for the oxidation of propylene over A2i3xBi J(f>xMo0.i, (A2+ = Pb, Cd, or Ca), compositions showed that when x = 0, the activity was very low but increased rapidly with increasing defect concentration. When bismuth was absent, the activity and selectivity were very poor on comparison with bismuth containing defect scheelites. 

The nonlinear optical oxide crystals recently developed are grown by flux (and hydrothermal solution for KTP) techniques to prevent decomposition (KTP, KTA, LBO) or to obtain a low temperature phase (BBO). The intrinsic nonstoichiometry and the impurity contents of the as-grown crystals is determined by the solutions and temperatures used for growth. The intrinsic defect concentrations in these materials are relatively low, compared to the more traditional nonlinear optical oxides having the... 

The p0 dependence of oxygen nonstoichiometry (8) was determined by using coulometric titration. The data were analyzed using a simple point defect model and thermodynamic quantities were calculated. From this model, the standard enthalpy for oxidation (AH0f) and disproportionation (A77D) were determined to be -140.7 and 228.7 kJ/mol, respectively. The mobilities of the electron holes, electrons, and oxygen ions were calculated from the conductivity data using the defect concentrations determined from the stoichiometry and point defect model. 

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