Aliovalent charge states

Aliovalent charge states can be observed in frozen aqueous solution of Co(II)-salts, where the formation of Fe can be explained by the radiolysis model. In this model, oxidation of the nucleogenic Fe to Fe is assumed by OH radicals, which form during the radiolysis of water molecules coordinated to the parent Co ion, by Auger electrons (Friedt and Danon 1980). 

V) The EC decay of Co may result in aftereffects however, changing the lattice site (for another one best suited for iron) is not expected. Only electronic changes may be found due to slow relaxation of electronic excitations or competing reactions between lattice defects and mobile charges. These can result in detecting, for example, aliovalent charge states of iron. 

Vi) In the systems discussed in this chapter, electron mobility is high therefore, the appearance of aliovalent charge states or any other species due to slow relaxation is not expected. 

When dealing with aliovalent foreign elements it is common to assume directly the ionisation of the donor or acceptor into an effectively charged state. We often refer to the cases as donor- or acceptor-doped systems, but keep in mind that it is the unionised species that constitutes the donor or the acceptor. 

Let us now consider the addition of small amounts of imperfections or impurities. These may be considered to contribute additional localised energy levels in the crystal. It is commonly assumed that these always fall within the forbidden gap. As we shall see later on this leads to situations where a certain temperature is needed to excite these defects to become effectively charged. This is not in accordance with experimental evidence and is not intuitive for imperfections with very stable aliovalent valence states. Thus, we will first consider cases where imperfections introduce levels outside the forbidden gap. 

For doped metal oxides, the aliovalent metal ions influence the overall defect concentrations via what is sometimes called the First Law of Doping. This law simply states that adding an aliovalent dopant increases the concentration of defects with opposite charges, and decreases the concentration of defects with charges of the same sign. 

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