As long ago remarked by WADSLEY, perovskite-like solids have a clear tendency to be non-stoichiometric and both the geometrical requirements of cation coordination and the oxidation state can control the way in which non-stoichiometry is accommodated.
For example, in the case of iron, which shows a particular preference for coexisting octaedral and tetrahedral coordination and in which oxidation states ranging formally from 2 + to 4 + are common in oxides, a Family of Phases AnFenO3n−1, has been described. It includes the members n=2,3,4 and ∞. When the stoichiometries are different from these integral n values, disordered 1−D intergrowthhs are observed. Subsequent work has shown that all the intermediate compositions, whether ordered or disordered in 1−D, can still become non-stoichiometric by oxidation at high temperatures (T>1200°C in air) and this originates the formation of 3D microdomains. Of these, up to nine different sets have been shown to coexist in the same crystal.
Similarly, the copper based HTSC, which have a lot in common with perovskite, have shown a marked tendency for being oxygen deficient. In particular, Ybacuo, Ba2YCu3O7±6 has been shown to exist in a wide oxygen range extending from, at least RO7.2 to RO6 (R<>Ba2YCu3). Within that range, and in spite of an apparent structurally monophasic, and thermodynamically bivariant, behaviour, a succesion of ordered states of the oxygen vacancies in 1,2 and 3 dimensions has been found. This, again, suggests the existence of intermediate ordered phases which, on the other hand, can also be non-stoichiometric. Results of our work along these lines is presented and discussed.