MICROSTRUCTURAL CHARACTERIZATION OF THE FLUORITE PHASE IN THE U-LA-O SYSTEM .2. HEXAGONAL MICRODOMAIN FORMATION IN (U1-YLAY)O-2-X, 0.70-LESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-0.80

Fluorite-type materials (U1-yLay)O2-x within the compositional range 0.70 ≤ y ≤ 0.80 have been characterized by X-ray diffraction and transmission electron microscopy. Their microstructures are discussed in terms of microdomain formation of a secondary phase, denominated RII coherently intergrown with the fluorite matrix. Annealing of the materials y = 0.73 and y = 0.74 at 1100°C for 500 hr leads to the formation of the RII phase as a single phase, which may be described as a superstructure 2ac × 2ac × 3ac of the fluorite cell. A new set of lattice parameters has been determined on the basis of a primitive trigonal cell, being ah = 0.78220(4) nm and ch = 0.99712(7) nm for the composition y = 0.74. Relationships between the fluorite and the RII cells are presented. © 1994 Academic Press, Inc.