The broadhyphen;line proton magnetic resonance spectrum of polycrystalline samples of the compound formulated empirically as UO42H2O has been investigated in the temperature range 77deg; to 356deg;K. Analyses of the absorption line shapes, and their derivatives, at 77deg; and 257deg;K, coupled with chemical analysis for peroxide oxygen, show that the compound is essentially the dihydrate of uranyl peroxide, UO2(O2)middot;2H2O. The sample investigated has about 1/12 of its protons in UOH or UOOH groups of a second component, possibly a hydroperoxide, UO(OH)3(OOH), or hydroxy peroxide, UO(OH)2(O2)middot;H2O. At temperatures between 125deg; and 210deg;K, the absorption of the OH protons in this component undergoes changes which are compatible with selfhyphen;diffusion. The apparent activation energy for this motion is 2 kcal. The two component formulation of the ``compound'' accounts for most, if not all, of its complex chemical behavior.
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