Hyperstoichiometric Oxygen in Fluorite-type U$_{3}$O$_{8}$ Formed at Extreme Conditions

U$_{3}$O$_{8}$ was obtained by annealing UO$_{3}$ in a reducing atmosphere at 200 $^{\circ}$C. Powder sample of $\beta $-U$_{3}$O$_{8}$ was pressurized at room temperature up to 37.5 GPa and XRD patterns clearly indicated that a phase transition occurred between 3-11 GPa. The high-pressure phase is a fluorite-like structure. The high-pressure phase was then laser heated to over 1700 K in the diamond anvil cell at high pressure conditions. No phase transition was found at high pressure/ temperature conditions, and the fluorite-like structure of U$_{3}$O$_{8}$ is even fully quenchable. The lattice parameter of the fluorite-like high-pressure phase is 5.425 {\AA} at ambient conditions, which is smaller than that of the stoichiometric UO$_{2}$. Previous experiments have shown that the stoichiometric uranium dioxide (UO$_{2})$ is not stable at high pressure conditions and starts to transform to a cotunnite structure at $\sim$ 30 GPa. When heating the sample at high pressure, the critical transition pressure is greatly reduced. However, the fluorite-like high-pressure phase of U$_{3}$O$_{8}$ is very stable at high pressure/high temperature conditions. The enhanced phase stability is believed to be related to the presence of extra oxygen (or U vacancies) in the structure.