MICROSTRUCTURAL CHARACTERIZATION OF THE FLUORITE PHASE IN THE U-LA-O SYSTEM .1. RHOMBOHEDRAL MICRODOMAIN FORMATION IN (U1-YLAY)O2+/-X1 0.56 LESS-THAN-OR-EQUAL-TO Y LESS-THAN-OR-EQUAL-TO 0.67

A series of hyperstoichiometric, stoichiometric, and hypostoichiometric ULa mixed oxides with fluorite-type structures has been prepared in air at 1673 K in the composition range (U1-yLay)O2±x, 0.33 ≤ y ≤ 0.67, by a mixed organic precursor/ceramic procedure. The oxidation state of uranium in these materials has been examined by XPS and near FT-IR spectroscopy, uranium ions existing within each sample either as U5+ and U6+ (0.33 ≤ y ≤ 0.50) or U6+ and U4+ (0.56 ≤ y ≤ 0.67). Electron diffraction studies reveal two sets of patterns with diffuse scattering phenomena depending on composition. The diffuse scattering for specimens with 0.56 ≤ y ≤ 0.67 is discussed in terms of microdomain formation of a rhombohedral RIII phase, that is obtained as a single phase when samples with these compositions were annealed at 1373 K for 100 hr. © 1990.