Oxygen excess nonstoichiometry

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

Anion-excess nonstoiciiiometry in perovskite oxides is less common than anion-defident nonstoichiometry, most likely because the introduction of interstitial oxygen into a perovskite structure is thermodynamically unfavorable. For example, careful stuches of system LaMnOa(including neutron diflraction) have revealed that an oxygen excess is accommodated by vacandes at the A- and B-sites, and in reality the compound with the proposed composition of LaMnOa 12 should be described as Lao.94 o.oeMno.gs nootOa [83]. 

From this equation it is qualitatively seen using le Chateher s principle that the oxygen deficit increases with decreasing oxygen pressure. Conversely, for oxides with excess oxygen the nonstoichiometry increases with increasing oxygen pressure. 

Nonstoichiometry in perovskite oxides can arise from cation deficiency in A or B sites or oxygen deficiency or oxygen excess. A classical example for A-cation deficient perovskites is tungsten bronzes, A WOs. In A WOs, A-cations (typically alkali ions) can be missing either partially or wholly and the resulting structures are therefore intermediate between the ABO3 perovskite structure and the Re03 structure. 

The nonstoichiometry, which indicates the cation deficiency in the sites A and/or B, as well as of oxygen, is a very common feature of perovskites, different from the ideal structure. In the case of oxygen site, nonstoichiometry obtained by the presence of vacancy is more common than that generated by the cationic vacancies. Therefore, the most common nonstoichiometiic structures are those with a relative excess of cations due to anionic vacancies. On the other hand, the excess of oxygen is not common, probably because the extra oxygen introduced in the network is thermodynamically unfavorable. 

The nonstoichiometry may be found on the oxygen sites as is the case with stabilized zirconia (Yj Zri j )02 j/2, metal deficient oxides such as Fei j 0, anion excess oxides such as UO2+J and systems that display combinations of these effects. The vacancies in such systems may cluster or ultimately may order by some mechanism such as crystallographic shear. Catlow has discussed such systems in detail. ... 

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. 

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