Search

While extensive success has been gained in the structural chemistry of the U-Se system, the synthesis and characterization of Th-based Se structures are widely unexplored. Here, four new Th-Se compounds, α-Th(SeO3)2, β-Th(SeO3)2, Th(Se2O5)2, and Th3O2(OH)2(SeO4)3, have been obtained from mild hydrothermal or low-temperature (180-220 °C) flux conditions and were subsequently structurally and spectroscopically characterized. The crystal structures of α-Th(SeO3)2 and β-Th(SeO3)2 are based on ThO8 and SeO3 polyhedra, respectively, featuring a three-dimensional (3D) network with selenite anions filling in the Th channels along the a axis. Th(Se2O5)2 is a 3D framework composed of isolated ThO8 polyhedra interconnected by [Se2O5]2- dimers. Th3O2(OH)2(SeO4)3 is also a 3D framework constructed by octahedral hexathorium clusters [Th6(µ3-O)4(μ3-OH)4]12+, which are interlinked by selenate groups SeO42-. The positions of the vibrational modes associated with both SeIVO32- and SeVIO42- units, respectively, were determined for four compounds, and the Raman spectra of α- and β-Th(SeO3)2 are compared and discussed in detail. [ABSTRACT FROM AUTHOR]

Pentavalent uranium compounds are key components of uranium‘s redox chemistry and play important roles in environmental transport. Despite this, well‐characterized U(V) compounds are scarce primarily because of their instability with respect to disproportionation to U(IV) and U(VI). In this work, we provide an alternate route to incorporation of U(V) into a crystalline lattice where different oxidation states of uranium can be stabilized through the incorporation of secondary cations with different sizes and charges. We show that iriginite‐based crystalline layers allow for systematically replacing U(VI) with U(V) through aliovalent substitution of 2+ alkaline‐earth or 3+ rare‐earth cations as dopant ions under high‐temperature conditions, specifically Ca(UVIO2)W4O14 and Ln(UVO2)W4O14 (Ln=Nd, Sm, Eu, Gd, Yb). Evidence for the existence of U(V) and U(VI) is supported by single‐crystal X‐ray diffraction, high energy resolution X‐ray absorption near edge structure, X‐ray photoelectron spectroscopy, and optical absorption spectroscopy. In contrast with other reported U(V) materials, the U(V) single crystals obtained using this route are relatively large (several centimeters) and easily reproducible, and thus provide a substantial improvement in the facile synthesis and stabilization of U(V). [ABSTRACT FROM AUTHOR]

Single crystals of α-Th(SeO3)2 (I) and β-Th(SeO3)2 (II) are grown under hydrothermal conditions using Th(NO3)4, SeO2, and NaNO3 (for intentional Na-containing compounds) in molar ratios of 1:3:5 and 1:1:3, resp. [ABSTRACT FROM AUTHOR]

Four new uranyl arsenates, Cs2[(UO2)(As2O7)] (1), α-Cs[(UO2)(HAs2O7)] (2), β-Cs[(UO2)(HAs2O7)] (3), Cs[(UO2)(HAs2O7)]·0.17H2O (4), were synthesized by high-temperature/high pressure (HT/HP) reactions at 900 °C and 3 GPa. These phases were subsequently characterised structurally as well as chemically. We demonstrated that compound 1 can also be obtained at ambient pressure. Compounds 1, 2, and 4 are based on two-dimensional (2D) anionic layers with two different topological types. The layers possess a similar composition, [(UO2)(As2O7)]2− in 1 and [(UO2)(HAs2O7)]− in 2 and 4. However, the presence of hydrogen in 2 and 4 leads to a change in coordination modes of the pyroarsenate groups. There are additional 0.17 H2O molecules per formula unit in 4, which cause slight distortions of the layers in 4. All these layers can be simplified to a common net, which is typical of autunite-like layered compounds. Compound 3 is a polymorph of compound 2, but the structural arrangements in these two are significantly different. The structure of 3 is based upon a three-dimensional (3D) framework, in which UO7 is coordinated by arsenate groups in order to form uranyl anion sheets, and UO6 is located within the interlayers. Bond valance analysis proved the presence of OH− groups in compounds 2, 3, and 4, respectively, and water molecules in 4. The Raman analyses enabled the study of the local environments of the arsenate and the uranyl groups within the investigated phases, respectively. It turned out that the applied HT/HP synthesis method strongly affects the crystal chemistry as well as the observed structural features of all obtained compounds. [ABSTRACT FROM AUTHOR]