[en] Work under Contract E(11-1)-1109 and its antecedents has been primarily for the purpose of obtaining detailed thermodynamic, kinetic and structural information on the complex rare earth oxides of praseodymium and terbium. These systems exhibit homologous series of ordered phases, order-disorder transformations, wide-range nonstoichiometric phases, chemical hysteresis in two-phase regions and many other solid state reaction phenomena. Fluorite-related materials of importance to ERDA occur as nuclear fuels, radiation power sources, insulators and solid electrolytes. The rare earth oxides serve directly as model systems for such similar materials and, in a more general sense, they serve as models of solids in general since they exhibit nearly the full range of solid state properties

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[en] Electron microscope images obtained at about 3A resolution for Pr₇O₁₂ (and isomorphous Zr₃Sc₄O₁₂) are in excellent agreement with calculated images based upon the structure determined separately by total profile neutron powder diffraction analysis. The image calculation programs establish the efficacy of lattice image interpretation in structural terms for these types of related phases whose structures are as yet beyond conventional means to determine. Such images for several other intermediate phases are interpreted and structures proposed. Finally, images of crystals which can be made to undergo reaction within the microscope are interpreted according to the structures involved. These studies reveal the role of intergrowth and topotaxy in these systems

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[en] The binary oxides of the rare-earth elements are, except for the A- and B-type sesquioxides, members of a fluorite-related homologous series R/sub n/O/sub 2n-2/ (n = 4, 6, 7, 9, 10, 11, 12, and infinity are well established and a related phase with n =10¹/₃ has been reported). In this paper an electron optical study of members of the series is discussed which reveals the unit cell dimensions and possible space groups of the intermediate phases as well as the transformation matrices in terms of the fluorite substructure. This information reveals the structural relationships among the members of the series as well as the highest common structural feature involved. Structures are proposed for members of this series consistent with the new results obtained and with the data already in existence. (U.S.)

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