THE BINARY HIGHER OXIDES OF THE RARE-EARTHS

Previously published thermodynamic and some structural studies of the higher oxides of cerium, praseodymium and terbium have revealed complex systems that include homologous series of ordered intermediate phases between R2O3 and RO2 at lower temperatures, two widely non-stoichiometric phases at higher temperatures, order-disorder transitions and many other somewhat mysterious solid state phenomena. These oxygen-deficient fluorite-related systems are the most elaborate of this structure type. One of the most important requirements for advancement of understanding of these complex binary oxide systems is a knowledge of the structures and structural principles that relate the members. Recently, high resolution neutron diffraction data have been collected from Tb7O12, Pr9O16 Pr10O18, Tb11O20 and Pr12O22 and their structures have been solved by Rietveld analysis. This makes it possible to advance the outlines of the structural principle involved in understanding the relationships among these fluorite-related phases. In addition, some discussion of the disorder-order transition will be given together with a study of the phases formed when the disordered alpha phase is cooled rapidly.