Structural features of rare earth oxides

Abstract

Non-stoichiometry is a phenomenon in solid compounds related to defects and mixed valence. Fifty years ago a wide-range of non-stoichiometric phases were interpreted in terms of homologous series of ordered intermediate oxides. Small oxygen content variations in the dioxides of the binary rare earths lead to oxygen-deficient, fluorite-related phases [1-4]. The assimilation of oxygen vacancies establishes ordered superstructures to the fluorite type crystal and is characteristic for the non-stoichiometric higher oxides of the binary rare earths [5]. These oxides occur in several oxidation states of which the trivalent is the most common. Based on the structure solution of numerous intermediate binary rare earth higher oxides during the last decade (see section 3.2.2) several review articles were published, summarizing [6] and interpreting the results using different approaches to formulate a theory in order to predict the yet unknown defect distributions of fluorite related anion deficient compounds in the system [7-10]. For several reasons it is difficult to determine the structures of the homologous series phases by X-ray diffraction due to the weak scattering of X-rays by oxygen atoms. Neutron diffraction turned out to be the best method to determine the structures of these ordered oxygen deficient phases but it is almost impossible to prepare a sample consisting of a single phase because the oxygen content of the sample will easily vary with temperature and oxygen partial pressure. However the refinement of the neutron diffraction data from a powder sample needs proposed models. Those models might come from the knowledge about a structural principle for the homologous series phases. The crux of solving these structures is to obtain the diffraction data from a monophase sample. To overcome this problem the use of high-resolution electron microscopy (HREM) is suitable in order to unravel the real structures in the different materials. The following article will present the efforts undertaken during the last decades, utilizing all three different diffraction methods and HREM, in order to elucidate the structures of the binary rare earth oxides. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.