Low‐Temperature Equilibria Among ZrO2, Tho2, and UO2

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Abstract
Studies made on low‐hafnium‐content ZrO2, show that the monoclinic‐tetragonal inversion temperature is 1170°C., and it is raised to approximately 1190°C. in the “natural” ZrO2, which contains approximately 2% HfO2. No explanation could be found for the knownmarked hysteresis during cooling, when the reverse polymorphic transformation takes dace at 1040°C. In the system ZrO2‐ThO2 the monoclinic‐tetragonal ZrO2, inversion temperature is lowered to 1000°C., although the maximum solid solution extent of ZrO2, 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 ThO2, to nearly pure ZrO2. Contrary to earlier work only 8 mole ZrO2, dissolves in UO2 and less than 4 mole of UO, in ZrO2 at temperatures up to 13OO0C. A continuous series of solid solutions could be made between Th2 and UO2 at 13OO°C., and extensive defect fluorite solid solutions could be prepared between Tho2 and U3O8; there is some evidence for exsolution into uranium‐rich and thorium‐rich members at low temperatures.