Hydrothermal Synthesis, Study, and Classification of Microporous Uranium Silicates and Germanates

Four novel uranyl silicates and germanates with framework structures, K₄Na₂(UO₂)₃(Si₂O₇)₂·3H₂O, K₄Na₂(UO₂)₃(Ge₂O₇)₂·3H₂O, H₃O(UO₂)₂(HGe₂O₇)·2H₂O, and Na₂(UO₂)GeO₄, have been synthesized by means of the hydrothermal method. The structures of the title compounds were refined by single-crystal X-ray diffraction and characterized by Raman spectroscopy. We used the method of secondary building units (SBUs) for a crystal chemical analysis of the 3D framework and their topologies. The framework of the K₄Na₂(UO₂)₃(T₂O₇)₂·3H₂O (T = Si, Ge) series exhibits large 14-membered rings and smaller 8-membered rings which are built upon [UT₄] pentamers. The internal size of the largest pores is approximately 12.39 × 3.33 Ų. H₃O(UO₂)₂(HGe₂O₇)·2H₂O is based on 10-membered rings with intermediate sized pores. They are built upon [U₂Ge₂] tetramers with 7-fold-coordinated U. The internal dimension of the pores in H₃O(UO₂)₂(HGe₂O₇)·2H₂O is smaller compared to the K₄Na₂(UO₂)₃(T₂O₇)₂·3H₂O (T = Si, Ge) series with ∼5.91 × 5.33 Ų. Its topology is similar to several uranium germanate synthetic phases and silicate minerals, especially α- and β-uranophane which are constructed from similar building units. A novel 3D framework type of Na₂(UO₂)GeO₄ with 8-membered rings demonstrates the smallest free volume in the family of porous uranium germanates. It crystallizes in tetragonal symmetry and is built upon corner sharing of [UGe₄] pentamers. The size of the channels is ∼6.76 × 4.27 Ų. The vibrational bands in Raman spectra were associated with pyro-(Si₂O₇)^6- and -(Ge₂O₇)^6- groups, with the Ge-OH bond and with H₃O⁺ cations, confirming the results of the X-ray crystallographic structural characterization. We systemized existing uranyl silicates and germanates based on their building units and chemical composition. We found a simple structural dependence between synthetic conditions and chemical composition.