Microstructural characterization of the fluorite phase in the U-La-O system

Fluorite-type materials (U[sub 1[minus]y]La[sub y])O[sub 2[minus]x] within the compositional range 0.70 [le] y [le] 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 R[sub II], coherently intergrown with the fluorite matrix. Annealing of the materials y = 0.73, and y = 0.74 at 1100[degrees]C for 500 hr leads to the formation of the R[sub II] phase as a single phase, which may be described as a superstructure [radical]2a[sub c] [times] [radical]2a[sub c] [times] [radical]3a[sub c] of the fluorite cell. A new set of lattice parameters has been determined on the basis of a primitive trigonal cell, being a[sub h] = 0.78220(4) nm and c[sub h] = 0.99712(7) nm for the composition y = 0.74. Relationships between the fluorite and the R[sub II] cells are presented.