Nonstoichiometry oxygen excess phases

Fluorite type oxides are particularly prone to nonstoichio-metric effects. This most commonly occurs in the form of cation nonstoichiometry induced by partial reduction of the cation or by replacement of a portion of the oxide by flnoride. Anion excess phases can occur as a result of cation oxidation or by replacement with higher valence impurities. The dominant defect in this structure involves the migration of oxygen to the large cuboidal interstice resulting in the formation of a vacancy at a normal lattice site. A vacancy of this type is called a Frenkel defect. 

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. 

The perovskite stracture tolerates relatively large nonstoichiometries and substitutions, while a number of perovskite-related stmctures (with perovskite building blocks) are formed when excesses, deficiencies or substitutions beyond the solubility limits lead to ordered defects and new phases. These comprise, for instance, oxygen deficiency (brownmillerite, ABO2.5), and A excess (K2NiF4-type structure, A2BO4) and ordered intermediate phases. 

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