Formation and Properties of Strontium Uranates

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Abstract
The formation and nonstoichiometry of strontium uranates, especially those with the Sr/U atom ratio around unity, were examined by means of thermogravimetry and X-ray diffraction. The reaction of SrCO3 with U3O8 in air, where Sr/U=1, produced the stoichiometric β-SrUO4, which had an orthorhombic structure. The reactions in vacuum and in hydrogen yielded the products of the composition SrUO3.563 and SrUO3.175, respectively. On the other hand, the reduction of β-SrUO4 in vacuum and in hydrogen gave SrUO3.48 and SrUO3.65, respectively. Stoichiometric SrUO3 was prepared by the reaction of SrO with UO2. The reduction product of β-SrUO4, SrUC4−x, was oxidized in air below 500 °C to α-SrUO4 which was rhombohedral. During the phase transition to β-SrUO4, α-SrUO4 exhibited anomalous behavior; that is, α-SrUO4 was first reduced and then reoxidized to nearly the initial composition with the formation of β-SrUO4. Of the uranates with Sr/U\eweq1, the formations of SrU4O13, SrU2O7, and Sr2UO5 were examined; the single phase SrU2O7 was not obtained. Thermograms for their reduction, followed by the oxidation of the resultants, suggest the existence of the compounds SrU4O11 and SrU2O6.0−6.6. No uranates containing only U(IV) were obtained by hydrogen reduction of the uranates containing U(VI) formed in air at high temperatures.

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