New aspects of uranium and thorium structural chemistry in oxo-silicate and oxo-germanate systems

The systematic study of the synthetic and coordination chemistry of novel actinide bearing oxo-silicates and oxo-germanates in respect to periodic trends between oxo-silicates and oxo-germanates was performed in this dissertation work. The novel phases were analyzed and characterized by SCXRD, PXRD, SEM-EDS, TG-DSC, Raman and IR spectroscopy methods. Moreover, this doctoral research has also focused on the analysis of some crystal chemical trends in all known U/Th oxo-salt phases. It was made for building of the relationships between synthetic conditions, chemical composition and crystal structures for some classes of existing uranium and thorium phases. The efforts may potentially be used for prediction of structural features of novel actinide bearing phases. The concept of secondary building units (SBUs) was widely used in this work for a crystal structural analysis. The existing uranyl silicates and germanates have been systemized based on their SBUs and chemical composition. Simple structural dependence between synthetic conditions and chemical composition has been found. Additionally, the SBUs combined with the cutting and gluing strategy was applied to understand the relation between porous multi-membered channel structures in uranium germanate family. The triangle diagram for all known A${}^{+}$–UO${}_2$${}^{2+}$–SiO${}_4$${}^{4-}$ phases demonstrates the high polymerization level of silicate groups in the system, which was compared with the family of A${}^{+}$–UO${}_2$${}^{2+}$–BO${}_3$${}^{3-}$/BO${}_4$${}^{5-}$ compounds. Furthermore, analysis of charge density for the U–Si–O system indicates that the polymerization of silicate units reduces the cross-links of the 3D frameworks. The charge density of all known 3D U–Si/Ge–O frameworks has been investigated, which shows a strong correlation with chemical composition of corresponding phases. This correlation can be used to predict inclusion phase formation within uranyl oxo-silicates/germanates families. For thorium silicate and germanate families, a number of phases including K${}_2$ThT${}_2$O${}_7$, K${}_2$ThT${}_3$O${}_9$ and Cs${}_2$ThT${}_6$O${}_{15}$ (T = Si, Ge) was described, and the phase transformation from K${}_2$ThSi${}_2$O${}_7$ to K${}_2$ThSi${}_3$O${}_9$ was uncovered. The coordination environment of U(VI) and Th(IV) for all existing oxo-anion compounds have been summarized and analyzed. The trend shows a strong dependence between the ionic potential of central cations (Si/Ge, P/As, Cr/Mo/W and S/Se/Te) in oxo-anions with total coordination number of U/Th.