Cationic defects, oxygen transfer

A wealth of information concerning the identities, mobilities, concentrations and properties of defects in many oxides is available through measurements of mass transfer made in association with metal oxidation studies [8,9]. The influences of crystal structure [10], temperatine and oxygen partial pressine on cation and anion migration have also been investigated. Information on the reactivities of oxides, their polymorphism and the properties of the imperfections present, is often useful in the formulation of the mechanisms of oxide dissociation. 

All these compounds are thought to possess neither non-stoichimetric reduced phases, nor extended defects, but rather point defects, mainly cation and anion vacancies. The latter are known to produce a considerable mobility in the lattice (5 ) In these compounds, the defect structures readily account for the rapid reoxidation of the bulk by rapid diffusion of oxygen and electron transfer, as well as for the ability of the host matrix to form coherent interfaces. 

Figure 11.10 shows the formation of p-type metal deficit oxides nonstoichiometric oxides with metal deficits. The defects are cation vacancies and oxygen interstitials. They are compensated through the formation ofpositive electronic defects (electronic holes). Electron charges are transferred to the metal-oxide interface by the movement of electron holes in the opposite direction. Metal cations diffuse at the oxide-air-gas interface, while the cation vacancies diffuse in opposite direction. The oxide film is formed at the oxide-metal interface. 

As described in the previous chapter, the Schottky disorder involves the presence of equivalent amounts of cation and anion vacancies. In an oxide MO this means that the erystal contains equal concentrations of metal and oxygen vacancies. The overall formation of such a defect pair within the crystal involves the transfer of a pair of cations and anions on regular lattice sites from the bulk to the surface. In reality the defects are formed at external and internal surfaces or... 

Write a reaction for a charge transfer between the cation and anion in Ce02, i.e. for reduction of the cerium ion and oxidation of the oxygen ion. Write the same process as an intrinsic ionisation assuming delocalised electronic defects. 

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