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

Four novel uranyl silicates and germanates with framework structures, K4 Na2 (UO2 )3 (Si2 O7 )2 ·3H2 O, K4 Na2 (UO2 )3 (Ge2 O7 )2 ·3H2 O, H3 O(UO2 )2 (HGe2 O7 )·2H2 O, 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 K4 Na2 (UO2 )3 (T2 O7 )2 ·3H2 O (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 Å2 . H3 O(UO2 )2 (HGe2 O7 )·2H2 O is based on 10-membered rings with intermediate sized pores. They are built upon [U2 Ge2 ] tetramers with 7-fold-coordinated U. The internal dimension of the pores in H3 O(UO2 )2 (HGe2 O7 )·2H2 O is smaller compared to the K4 Na2 (UO2 )3 (T2 O7 )2 ·3H2 O (T = Si, Ge) series with ∼5.91 × 5.33 Å2 . 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 Å2 . The vibrational bands in Raman spectra were associated with pyro-(Si2 O7 )6- and -(Ge2 O7 )6- groups, with the Ge-OH bond and with H3 O+ 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.