Grossly non-stoichiometric oxides

Investigations based on equation (a) are indirect. Direct structural studies using diffraction techniques (X-ray or neutron), or electron microscopy, while they cannot detect the low concentrations of defects present in NiO or CoO are indispensible to the study of grossly non-stoichiometric oxides like FeO, TiOj, WOj etc., and particularly electron microscopes with a point-to-point resolution of about 0.2 nm are widely used. The first direct observation of a point defect (actually a complex of two interstitial metal atoms, and two oxygen atoms in Nb,2029) was made" using electron microscopy. 

Configurational averaging - solid solutions and grossly non-stoichiometric oxides... 

The same general approach can be applied to grossly non-stoichiometric oxides [8], For an oxygen deficient perovskite such as SrFeC>2.5 this involves an explicit... 

The point defects discussed above exist as such, only at low enough concentrations (perhaps up to 0.5% depending on the material). At higher concentrations the point defects can aggregate into clusters, or become ordered, or can be eliminated by forming 2- or 3-dimensional defects such as shear planes, or voids. In grossly non-stoichiometric oxides such as reduced Ti02, VO2, CrOj, WO3 and MoOj, simple point defects may be in the minority. 

This area has been widely investigated since the discovery by Zintl and Croatto (1939) of the existence of so-called anomalous or heterotype mixed crystals in the system CeOz-LazOs. These grossly non-stoichiometric phases, which often have wide compositional ranges, are virtually unique in oxide systems, and some have the important property of good anionie conduction (Mobius, 1964). However, in spite of much research effort there is still considerable disagreement on the precise nature of the phase equilibria in such systems. 

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