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Results show that PuO2+x, a high-composition (x ≤ 0.27) phase containing Pu(VI), is the stable binary oxide in air. This nonstoichiometric oxide forms by reaction of dioxide with water and by water-catalyzed reaction of dioxide with oxygen. The PuO2 + H2O reaction rate is 0.27 nanomoles per meter squared per hour at 25°C; the activation energy at 25° to 350°C is 39 kilojoules per mole. Slow kinetics and a low lattice parameter–composition dependence for fluorite-related PuO2+ x are consistent with a failure to observe the phase in earlier studies. Perplexing aspects of plutonium oxide chemistry can now be explained.
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