Thermodynamics and phase relationships of the ternary lanthanum-uranium-oxygen system

In the selected regions La:(La + U) = 0.05 and O:(La + U) = 2.00 of the ternary system lanthanum-uranium-oxygen emf measurements on solid state galvanic cells, coulometric titrations, and X-ray diffraction techniques were used to obtain phase boundaries and thermodynamic data in the temperature range from 600 to 1000°C. For the first time order disorder transformations of La_1−yU_yO_2+x up to 15 mole% lanthanum are reported. The transformation temperature is 1415°K for UO_2.23; 1397°K for La_0.05U_0.95O_2.23, and 1449°K for La_0.15U_0.85O_2.23. The vibrational entropy component of excess oxygen in M_1−yU_yO_2+x is estimated.