Substitution of Ca for La ions in the U-Ca mixed oxides: effects on thermal stability of the fluorite phase

Two series (A and B) of U-Ca-La oxides have been prepared in air at 14000C. Series A corresponds to the composition range (U/sub 0.5/Ca/sub 0.5-y/La/sub y/)O2μ- x, where 0 ≤ y ≤ 0.5, and series B corresponds to (U/sub 0.67/Ca/sub 0.33-y/La/sub y/)O/sub 2+x/, where 0 ≤ y ≤ 0.33 The obtained products have been characterized by X-ray powder diffraction, thermal analysis, and electron microscopy. In materials belonging to series B a f.c.c. fluorite-type structure is always obtained as a single phase over the whole composition range. Thermal stability up to 14000C is achieved when the lanthanum content in these samples ranges from 0.13 ≤ y ≤ 0.33. Electron diffraction patterns show diffuse maxima indicating the existence of systematic short-range deviations form the periodicity of the average unit cell. In series A, five well-defined regions are observed, but the fluorite structure is not formed