Anion deficiency in strontium titanate

Abstract

FRANCO and REGI's¹ diffraction patterns and lattice images of anion-deficient strontium titanate, SrTiO_3−x, show a sixfold superlattice along [111] and associated lattice images with irregular fringe spacings. The possibility of anion vacancy ordering was induced with reference to the systems (Ba,Sr) MnO_3–x (ref. 2) and BaFeO_3–x (ref. 3). These results contrasted with earlier data on SrTiO_3–x (0⩽x⩽0.5) which indicated^4,5 on the basis of powder X-ray diffraction, a simple cubic cell with virtually unchanging lattice constant over the whole range of x. Here we draw attention to two points concerning the nonstoichiometry of strontium titanate. First, unlike the manganate and ferrate of refs 2 and 3, SrTiO₃ is a simple cubic perovskite with cubic stacking of SrO₃ layers. Second, in contrast to the impression given, oxygen vacancy ordering had not been convincingly established at all until recently in any anion-deficient cubic-stacked perovskite. Examples of such nonstoichiometric systems are SrTiO_2.5–3.0 (refs 4, 5), SrVO_2.5–3.0 (ref. 4), SrFeO_2.78–3.0 (ref. 6) and Sr₂LiMoO_5.5 (ref. 7). I started to study the three strontium-transition metal oxide systems in the expectation that application of the more powerful diffraction techniques such as electron diffraction would reveal anion ordering processes possibly overlooked by X rays. This proved to be the case for the SrFeO_3–x system where an ordered phase, SrFeO_2.75, with superlattice reflections clearly apparent by electron diffraction, but not by powder X-ray diffraction, was found⁸.