Characterizing solution and solid-phase amorphous uranyl silicates

A combination of high-energy X-ray scattering (HEXS) and X-ray diffraction (XRD) is used to structurally and chemically characterize uranyl-silicate solutions and precipitates. Starting with a U to Si ratio of 1:2, solutions prepared at room temperature from pH 2.2 to 9.0 and at 150 °C from pH 5.1 to 9.1 showed U-U correlations out to distances of 10 Å or longer in both final solutions and precipitates. With one exception, all of the precipitates were amorphous, with no evidence of Bragg diffraction in the XRD data. The room temperature samples above pH 3.1 all had similar Fourier transforms of their HEXS data, which were obtained from suspended slurries or precipitates. In contrast, the hydrothermal sample precipitates showed considerable variation in their HEXS correlations at longer distances. The XRD pattern of the hydrothermal sample with a pH of 5.1 exhibited Bragg reflections indexable as soddyite. While showing no evidence of crystallinity using XRD, the hydrothermal sample at pH 6 showed similar HEXS correlations, which evolved in samples at increasing pH into correlations more consistent with sodium boltwoodite. These findings are supported by thermodynamic modeling. The structural correlations seen in the HEXS data to distances of about 4 Å are similar in all samples prepared at pH 4 or higher. This similarity of structure is used to propose a model for solid formation that includes a uranyl silicate building block, or synthon, which preorganizes in solution. Varying the pH changes how these synthons link into larger structures.