Low‐Temperature Equilibria Among ZrO ₂ , Tho ₂ , and UO ₂

Studies made on low‐hafnium‐content ZrO 2 , show that the monoclinic‐tetragonal inversion temperature is 1170°C., and it is raised to approximately 1190°C. in the “natural” ZrO 2 , which contains approximately 2% HfO 2 . No explanation could be found for the knownmarked hysteresis during cooling, when the reverse polymorphic transformation takes dace at 1040°C. In the system ZrO 2 ‐ThO 2 the monoclinic‐tetragonal ZrO 2 , inversion temperature is lowered to 1000°C., although the maximum solid solution extent of ZrO 2 , in Thon and vice versa is approximately only 2% at this temperature. Below about 400°C. under hydrothermal conditions it was possible to prepare a continuous, although metastable series of solid solutions with the fluorite structurewith compositions varying from ThO 2 , to nearly pure ZrO 2 . Contrary to earlier work only 8 mole ZrO 2 , dissolves in UO 2 and less than 4 mole of UO, in ZrO 2 at temperatures up to 13OO0C. A continuous series of solid solutions could be made between Th 2 and UO 2 at 13OO°C., and extensive defect fluorite solid solutions could be prepared between Tho 2 and U 3 O 8 ; there is some evidence for exsolution into uranium‐rich and thorium‐rich members at low temperatures.