The formation of fluorite-type phases was investigated in R2O3-U3O8-O2 systems (where R-La, Sm, Dy, Yb) annealed for 66-85 hr at 1200C, and the solubility limits of the rare earth oxides in uranium octoxide were determined. The U(VI) content and the total uranium content were determined by coulometric analysis at a controlled potential. X-ray phase analysis was carried out by using the powder method with RKD-57 and RKU-86 cameras. The degree of oxidation of uranium changes with the ratio R/U and reaches 6 in samples where R/U=2/1. Thus, the presence of a rare earth oxide increases the stability of the hexavalent state of uranium at high temperatures. Contrary to expectations, the solubility of rare earth oxides in U3O8 was found to be very low (less than 1.5 mol % of RO1.5). (Author).

Binary Rare Earth Oxides is the first book in the field of rare earth oxides that provides coverage from the basic science through to recent advances. This book introduces the unique characteristics of the binary rare earth oxides with their chemistry, physics and applications. It provides a comprehensive review of all the characteristics of rare earth oxides, essential for scientists and engineers involved with rare earths, oxides, inorganic materials, ceramics, and structures. The binary rare earth oxides bring us a variety of interesting characteristics. Understanding their fundamental mechanisms builds a bridge between solid-state chemistry and materials science. The book begins with a brief introduction to binary rare earth oxides, their physical and chemical stabilities, polymorphism, crystal structures and phase transformation and the association with current applications. The book goes on to present the band structure of the oxides using several quantum chemical calculations, which belong to a newly developed area in the binary rare earth oxides. Central to this chapter are the characterizations of electrical, magnetic and optical properties, as well as details of single crystal growth and particle preparation methods that have progressed in recent years. Later chapters concentrate on thermo-chemical properties and trace determination techniques. The final chapter contains a variety of useful applications in various fields such as phosphors, glass abrasives, automotive catalysts, fuel cells, solid electrolytes, sunscreens, iron steels, and biological materials. This book is an invaluable resource for materials scientists and solid-state physicists and chemists with an interest in rare earth oxides, as well as advanced students and graduates who require an approach to familiarize them with this field. This book provides guidance through a comprehensive review of all the characteristics of binary rare earth oxides.

The physical and chemical properties of UO2 nuclear fuels are affected as fission products accumulate during irradiation. The lanthanides, a main group of fission products, form extensive solid solutions with uranium oxide in the fluorite structure. Thermodynamic studies of such solid solutions had been performed to obtain partial molar free energies of oxygen as a function of dopant concentration and temperature; however, direct measurement of formation enthalpies was hampered by the refractory nature of these oxides. In this work, high temperature oxide melt solution calorimetry was utilized to study the thermochemistry of various rare earth doped uranium oxide Lnx̳U1−x̳O2−0.5x̳+̳y̳ (Ln = La, Y, Nd) over a wide range of dopant concentrations and oxygen contents. The sintered solid solutions were carefully characterized to determine their phase purity, chemical composition, and uranium oxidation state, with most of the materials in the oxygen excess regime. The enthalpies of formation of Lnx̳U1−x̳O2−0.5x̳+̳y̳ were calculated from the calorimetric data. The oxidation enthalpies of these solid solutions are similar to that of UO2. The formation enthalpies from constituent oxides (LnO1.5, UO2, and UO3) become increasingly negative with addition of dopant cations and appear relatively independent of the uranium oxidation state (oxygen content) when the type and concentration of the dopants are the same. This is valid in the oxygen excess regime; thus an estimation of formation enthalpies of Lnx̳U1−x̳O2 materials can be made. The formation enthalpies from elements of hyperstoichiometric Lnx̳U1−x̳O2−0.5x̳+̳y̳ materials obtained from calorimetric measurements are in good agreement with those calculated from free energy data. A direct comparison between the formation enthalpies from calorimetric study and computational research using density functional theory was also performed. The experimental and computational energies of Lnx̳U1−x̳O2 (Ln = La, Y, Nd) generally agree within 10 kJ/mol. Since all the other doped fluorite oxides based on zirconia, hafnia, ceria, and thoria are in the oxygen deficit (oxygen vacancy formation) regime, a systematic study of these rare earth doped fluorite oxides (Lnx̳A1−x̳O2−0.5x̳) was made comparing experimental and computational results. A consistent trend suggested by both calorimetry and computation, was found for all oxygen vacancy containing systems (actinide and non-actinide oxide systems). Larger size mismatch between the smaller host cation (A4+) and the larger rare earth dopant cation (Ln3+) generally produces more stable solid solutions. The energetics of these systems is the result of competition between strain energy arising from size mismatch (endothermic) and defect association (exothermic). The formation enthalpies of Lnx̳U1−x̳O2−0.5x̳ obtained from calculation are slightly positive.