Anion deficiency in strontium titanate

Abstract

FRANCO and REGI's1 diffraction patterns and lattice images of anion-deficient strontium titanate, SrTiO3−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) MnO3–x (ref. 2) and BaFeO3–x (ref. 3). These results contrasted with earlier data on SrTiO3–x (0⩽x⩽0.5) which indicated4,5 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 SrTiO2.5–3.0 (refs 4, 5), SrVO2.5–3.0 (ref. 4), SrFeO2.78–3.0 (ref. 6) and Sr2LiMoO5.5 (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 SrFeO3–x system where an ordered phase, SrFeO2.75, with superlattice reflections clearly apparent by electron diffraction, but not by powder X-ray diffraction, was found8.