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Structural characterization of natural UO₂ at pressures up to 82 GPa and temperatures up to 2200 K

Steeve Gréaux^1,*, Laurent Gautron^1,, Denis Andrault^2,3, Nathalie Bolfan-Casanova³, Nicolas Guignot^4, and Julien Haines⁵

¹ Laboratoire des Géomatériaux et Géologie de l’Ingénieur, Université Paris-Est Marne la Vallée, France
² Institut de Minéralogie et de Physique des Milieux Condensés, Paris, France
³ Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, France
⁴ European Synchrotron Radiation Facility, Grenoble, France
⁵ Institut Charles Gerhardt Montpellier, Université de Montpellier, Montpellier, France

Uranium is one of the main heat sources in the Earth, as about 25% of the total heat is produced by the radioactive decay of U. The location of U in the deep mantle is then essential for a better understanding of the geodynamics and thermal behavior of the Earth. For the first time, the crystal structure of natural simple dioxide UO₂ uraninite has been studied by X-ray diffraction with synchrotron radiation (ESRF, Grenoble, France), in situ in a laser-heated diamond-anvil cell at pressures and temperatures relevant to the deep Earth’s mantle. Fluorite-type UO₂ displays a new sequence of phase transitions at high P and T, with a cubic modified fluorite Pa observed at 18 GPa, and an orthorhombic Pbca structure from 33 GPa up to 82 GPa. Using a second-order Birch-Murnaghan equation of state, we calculated room-pressure bulk modulus K₀ = 166(7) GPa with pressure derivative K'₀ = 4.0 for the Pa structure, and K₀ = 225(8) GPa with K'₀ = 4 for the Pbca structure. The expected Pnma cotunnite structure was not observed but is not excluded at pressures higher than 82 GPa. Since UO₂ displays a Pbca structure stable up to 82 GPa and presents a density much higher than the average density of the surrounding mantle, UO₂ could be a host of U in the deep lower mantle.

Key Words: Heat sources • uranium oxide • X-ray diffraction • crystal structure • deep mantle