Phase relationships for the ternary UO 2UO 2REO 1.5 (RE = Rr, Nd, Dy) systems

Abstract

Investigations of the phase equilibria in the oxygen-rich part of the ternary systems UREO with the light lanthanides praseodymium and dysprosium involving X-ray and EMF-methods have shown that the following single phase regions exist in all systems: a fluorite phase (U 1− y , RE y )O 2∓ x , a rhombohedral phase UO 3·6REO 1.5 and a C-type phase. The phase width in the two systems for the different single phases differs, however, especially for the rhombohedral phase. No solubility of REO 1.5 in U 3O 8 could be detected whereas REO 1.5 dissolves small quantities of UO 2 + x , the amount being higher the smaller the value of x. In vacuum the stoichiometric member of the fluorite phase, (Pr 0.5, U 0.5)O 2, is thermodynamically stable only at T > 1073 K. A detailed description of the phase diagrams of these systems is given together with thermodynamic data for the Partial Molar Gibbs Energy of oxygen in stoichiometric compounds of the systems with RE = Pr, Nd and Ce, obtained using a specially developed high-temperature galvanic cell.