Comparison of Uranium(VI) and Thorium(IV) Silicates Synthesized via Mixed Fluxes Techniques

Two uranium and two thorium silicates were obtained using high temperature mixed fluxes methods. K<sub>14</sub>(UO<sub>2</sub>)<sub>3</sub>Si<sub>10</sub>O<sub>30</sub> crystallizes in the <i>P</i>2<sub>1</sub>/<i>c</i> space group and contains open-branched sechser (six) single silicate chains, whereas K<sub>2</sub>(UO<sub>2</sub>)­Si<sub>2</sub>O<sub>6</sub> crystallizes in the <i>C</i>2/<i>c</i> space group and is built of unbranched achter (eight) silicate chains. The crystals of K<sub>14</sub>(UO<sub>2</sub>)<sub>3</sub>Si<sub>10</sub>O<sub>30</sub> and K<sub>2</sub>(UO<sub>2</sub>)­Si<sub>2</sub>O<sub>6</sub> are related by increasing U/Si molar ratios, and both structures contain the same secondary building units (SBUs), [USi<sub>6</sub>] heptamers. The triangle diagram for all known <b>A<sup>+</sup></b>–UO<sub>2</sub><sup>2+</sup>–SiO<sub>4</sub><sup>4–</sup> phases demonstrates the high polymerization level of silicate groups in the system, which was compared with the family of <b>A<sup>+</sup></b>–UO<sub>2</sub><sup>2+</sup>–BO<sub>3</sub><sup>3–</sup>/BO<sub>4</sub><sup>5–</sup> compounds. For both thorium silicates, the transformation of K<sub>2</sub>ThSi<sub>2</sub>O<sub>7</sub> to K<sub>2</sub>ThSi<sub>3</sub>O<sub>9</sub> was found to be a factor of the reaction time. K<sub>2</sub>ThSi<sub>2</sub>O<sub>7</sub> crystallizes in the <i>C</i>2/<i>c</i> space group and belongs to the Na<sub>2</sub>Si<sup>VI</sup>Si<sub>2</sub>O<sub>7</sub> structure type. Its 3D framework consists of diorthosilicate Si<sub>2</sub>O<sub>7</sub> group and ThO<sub>6</sub> octahedra. Noncentrosymmetric K<sub>2</sub>ThSi<sub>3</sub>O<sub>9</sub> crystallizes in the hexagonal <i>P</i>6<sub>3</sub> space group and adopts mineral wadeite-type structure based upon triorthosilicate Si<sub>3</sub>O<sub>9</sub> rings and ThO<sub>6</sub> octahedra. The coordination environment of thorium for all existing oxo-anion compounds including B, Si/Ge, P/As, Cr/Mo/W, and S/Se/Te are summarized and analyzed. Additionally, spectroscopic properties of all novel materials have been studied.