CHARACTERIZATION OF THE PLUTONIUM WATER REACTION .2. FORMATION OF A BINARY OXIDE CONTAINING PU(VI)

Characterization of products formed by reacting unalloyed plutonium with water vapor at 15 Torr pressure shows that Pu2O3, PuO2 and a previously unreported binary oxide containing Pu(VI) form on the metal between 200 and 350-degrees-C. Although the reaction produces a uniformly adherent product layer with a thickness of about 5 mum, localized pits of plutonium hydride cover 15-20% of the oxide-metal interface beneath the layer. Analytical results show that the non-equilibrium product contains layers of three oxide phases at all temperatures. Cubic alpha-Pu2O3 appearing at the metal interface is covered by a layer of dioxide. A third oxide formed at the gas-solid interface is identified by X-ray diffraction and X-ray photoelectron spectroscopy as the mixed-valence phase Pu(IV)3-xPu(VI)xO6+x. Diffraction data suggest that this oxide crystallizes in a fluorite-related NpH2+delta or Sm3H7(Ca2YF7)-type structure. Formation of the higher oxide is supported by X-ray photoelectron spectra, indicating the presence of the Pu(VI) oxidation state. Evaluation of the results yields an oxide composition corresponding to a value of x near 0.5. Estimated thermodynamic data suggests that the oxide is a stable phase in the Pu-O system; experimental observations show that the product is preserved during exposure to air and ultrahigh vacuum. Although formation of PuO2.2 is expected to be a slow process at low temperatures, reaction of PuO2 with moisture adsorbed on its surface may generate large quantities of hydrogen if dioxide is stored for an extended period.