Anion deficiency in strontium titanate

Abstract

STUDIES of the anion deficient phases of titanium and vanadium perovskites showed that the perovskite SrTiO3−x has a wide range of composition extending from x = 0 to x = 0.5 (ref. 1). The nonstoichiometry was inferred from the monophasic behaviour of the solid within this compositional range as obtained by means of powder X-ray diffraction. The same cubic structure was found for both Sr Ti O2.5 and Sr Ti O3. Perovskites have been widely studied recently in view of the very interesting and useful properties of these oxides2. The physiochemical properties of Sr Ti O3 include semiconductivity3,4 and superconductivity5 as well as peculiar phase transitions6. Of particular interest in this, and other7, systems is the presence of point defects and their influence on those properties. It seems that, at small degrees of reduction, doubly ionised oxygen vacancies are the main defects present on Sr Ti O3−x (ref. 8). Although in that case, the oxygen vacancies, being at small numbers, are probably distributed at random, the presence of so many empty oxygen positions as the stoichiometry of SrTiO2.5 implies—one in every six oxygens missing—automatically suggests the possibility of ordering. This possibility seems to be realised under the synthesis conditions that we have used and we report here some of the results so far obtained in the system SrO–Ti2O3–TiO2.