Oxygen nonstoichiometry component

Equation (56) indicates that when an oxygen atom is incorporated into the lattice, it picks up holes [Eq. (52)] and electrons [Eq. (53)] at random, depending on then-availability. It is, therefore, quite natural that the partial molar enthalpy of the component oxygen, AHo is dependent on the relative amount of electrons and holes or the oxygen nonstoichiometry. 

The catalytic activity of binary ABO3 perovskites can further be enhanced by partial substitution of lanthanum that can induce the oxygen nonstoichiometry with two possible consequences (i) the formation of a larger proportion of metal ions in unstable oxidation states, and (ii) an overall enhancement of oxygen mobility [32]. In addition, these catalysts exhibit a superior stability against deactivation by the additional components from the VOCs [34]. 

There has been considerable interest in Ce02 as a component of solid oxide fuel cells, especially as an anode material, and also for use in oxygen separation membranes. The material shows a wide nonstoichiometry range, with oxygen vacancies as the... 

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))... 

Nonstoichiometry can be caused by oxygen deficiency (or excess) or by fractional valences of the metal components. For example, the existence of Cu " in nonstoichiometric cuprates has been widely discussed [9,10]. It is essential that in nonstoichiometric oxides the microscopic fluctuations of the composition should proceed (the so-called phase separation). The characteristic size of heterogeneities induced can exceed atomic dimensions by an order of magnitude. This phenomenon is attributable to the fact that the electron-nonuniform state of such chemically singlephase materials appears to be energetically more advantageous. 

Figure 10.11 Relative partial molar enthalpy of component oxygen versus nonstoichiometry for undoped BaTiOs. The solid curve is best fitted to Eq. (50). Data from Ref. [19].

Although a particular type of defect predominates in an oxide and as such constitutes the majority defect in the crystal, it is important to emphasise that the crystal will contain all other defects in varying concentrations in the form of minority defects. Furthermore, the majority defects in the crystal may also change with temperature or activity of the components in the crystal. By way of example, in an oxide MO2 the predominant defects may in principle change from oxygen vacancies to interstitial metal ions on changing the oxygen activity, and under these conditions the nonstoichiometry may be written Mi+x02-x-... 

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