High-pressure equation of state of U$_{3}$O$_{8}$*

We will present experimental studies at high pressures of the equation of state of U$_{3}$O$_{8}$. Isothermal pressure-volume measurements of U$_{3}$O$_{8}$ were made at ambient/elevated (600 K) temperatures in the pressure range of 1 atm $\sim$ 80 GPa (10 $\sim$ 70 GPa). Angle dispersive X-ray diffraction patterns at ambient temperature indicate that the A-centered orthorhombic structure of U$_{3}$O$_{8}$ transforms to the face centered cubic (fcc) structure above 9 GPa. Both the orthorhombic and cubic phases co-exist between 9 GPa and 30 GPa. As the temperature is increased at 10 GPa, we find that U$_{3}$O$_{8}$ also transforms to the fcc structure. As the pressure is increased at 600 K, the fcc structure undergoes a phase transition to the body centered tetragonal structure. Since the uranium in U$_{3}$O$_{8}$ is the dominant x-ray scatterer, the behavior of the oxygen at the phase transitions was measured using Raman spectroscopy.