Characterizing solution and solid-phase amorphous uranyl silicates
Characterizing solution and solid-phase amorphous uranyl silicates
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DOI：
10.1016/j.gca.2007.10.002
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发表时间：
2008
影响因子：
5
通讯作者：
L. Soderholm;S. Skanthakumar;D. Gorman-Lewis;M. Jensen;K. L. Nagy
中科院分区：
文献类型：
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作者：
L. Soderholm;S. Skanthakumar;D. Gorman-Lewis;M. Jensen;K. L. Nagy
高能X射线散射（HEXS）和X射线衍射（XRD）的组合用于结构和化学表征铀酰硅酸盐溶液和沉淀物。以1：2的U与Si比开始，在室温下从pH 2.2至9.0和在150°C下从pH 5.1至9.1制备的溶液在最终溶液和沉淀物中均显示出10 μ m或更长距离的U-U相关性。除了一个例外，所有的沉淀物都是无定形的，在XRD数据中没有布拉格衍射的证据。pH 3.1以上的室温样品都具有相似的HEXS数据的傅立叶变换，其从悬浮浆料或沉淀物获得。与此相反，热液样品沉淀物显示出相当大的变化，在他们的HEXS相关性在较长的距离。pH为5.1的水热样品的XRD图案表现出可指示为方钠石的布拉格反射。虽然使用XRD没有显示结晶度的证据，但pH 6下的水热样品显示出类似的HEXS相关性，其在增加的pH下在样品中演变成与钠硼钙石更一致的相关性。这些发现得到了热力学模型的支持。在HEXS数据中观察到的距离约为4 μ m的结构相关性在pH 4或更高时制备的所有样品中相似。这种结构的相似性被用来提出一种固体形成的模型，其中包括一个硅酸铀酰结构单元，或合成子，它在溶液中预组织。改变pH值会改变这些分子连接成更大结构的方式。
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 10Å 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 4Å 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.