High-temperature X-ray study of uranium oxides in the UO2U3O8 region

Abstract Uranium oxides in the UO2U3O8 range have been studied by the X-ray powder method at temperatures between 20° and 969°C after annealing at 1000°C. The following phases were found: 1. 1. UO2+x with no detectable range of homogeneity at room temperature. The homogeneity range extends to about UO2·17 at 950°C. The lattice constant of UO2 is a = 5·4704 A at 20°C, and the average linear thermal coefficient of expansion α is 10·8 . 10−6 °C−1 between 20° and 946°C. The observed density of UO2·00 is 10·793 g.cm−3 at 25°C. 2. 2. UO2·25 or U4O9 with a narrow range of homogeneity. The lattice constant decreases from a = 5·4411 A at 20°C to 5·4397 A at 86°C. From then on it increases linearly to 960°C with α = 11·6 . 10−6 °C−1. The high density observed, 11·159 g.cm−3, shows that oxygen atoms are taken up interstitially in the UO2-like structure. 3. 3. U3O8−x with a range of homogeneity between UO2·62 and UO2·667 at room temperature. The lattice constants vary between these limits; UO 2·633 : a = 6·735 A , b = 3·966 A , c = 4·144 A . UO 2·667 : a = 6·720 A , b = 3·983 A , c = 4·146 A . The observed density of UO2·633 is 8·408 g.cm−3, and of UO2·667, 8·378 g.cm−3. The homogeneity range of the U3O8−x phase extends to about UO2·55 at 500° and 750°C.