Complex Topology of Uranyl Polyphosphate Frameworks: Crystal Structures of α-, β-K[(UO2)(P3O9)] and K[(UO2)2(P3O10)]

Targeted Synthesis of Uranium(IV) Thiosilicates

Two new uranium(IV) thiosilicates, Cs₂Na₄[U₂(SiS₄)₂(Si₂S₈)] and Cs_2.12Na_3.88[U₂(SiS₄)₂(Si₂S₇)], were obtained by flux crystal growth using SiS₂ as a silicon source. The former compound contains a novel Si₂S₈^6- unit that features a terminal persulfide group. The magnetic susceptibility measurements performed on this compound revealed paramagnetic behavior with a moment of 3.49 μ_B per uranium atom as obtained from a Curie-Weiss law fit and showed no magnetic transition down to 2 K. The structures are based on closely related isomeric planar and 3D topologies that can be transformed into one another by a rotation of the structural units.

Flux crystal growth: a versatile technique to reveal the crystal chemistry of complex uranium oxides

This frontier article focuses on the use of flux crystal growth for the preparation of new actinide containing materials, reviews the history of flux crystal growth of uranium containing phases, and highlights the recent advances in the field. Specifically, we discuss how recent developments in f-element materials, fueled by accelerated materials discovery via crystal growth, have led to the synthesis and characterization of new families of complex uranium containing oxides, namely alkali/alkaline uranates, oxychlorides, oxychalcogenides, tellurites, molybdates, tungstates, chromates, phosphates, arsenates, vanadates, niobates, silicates, germanates, and borates. An overview of flux crystal growth is presented and specific crystal growth approaches are described with an emphasis on how and why they - versus some other method - are used and how they enable the preparation of specific classes of new materials.

Formation of Open Framework Uranium Germanates: The Influence of Mixed Molten Flux and Charge Density Dependence in U-Silicate and U-Germanate Families

Seven novel open-framework uranyl germanates, K₂(UO₂)GeO₄, K₆(UO₂)₃Ge₈O₂₂, α-Cs₂(UO₂)Ge₂O₆, β-Cs₂(UO₂)Ge₂O₆, Cs₂(UO₂)GeO₄, and A(UO₂)₃(Ge₂O₇)₂ (A = [NaK₆Cl]⁶⁺, [Na₂Cs₆Cl₂]⁶⁺), were grown from different mixed molten fluxes. The three-dimensional (3D) structure of K₂(UO₂)GeO₄ with 8-ring channels can be built upon [UGe₄] pentamer secondary building units (SBUs). The 3D framework of K₆(UO₂)₃Ge₈O₂₂ with trapezoid (Ge₈O₂₂)^12- clusters consists of two types of [UGe₄] pentamers. The 3D framework of α-Cs₂(UO₂)Ge₂O₆ with 10-ring channels, crystallizing in the P2₁/ n space group, is constructed by [UGe₄] pentamers. The structure of β-Cs₂(UO₂)Ge₂O₆ contains achter (eight) single germanate chains and is composed of [UGe₆] heptamers and [UGe₄] pentamers. The structure of Cs₂(UO₂)GeO₄ with hexagonal 10-ring channels is composed of [U₃Ge₄] heptamers and twisting five-fold GeO₄ tetrahedra in four-membered Ge₄O₁₂ rings occur. 3D frameworks of NaK₆Cl(UO₂)₃(Ge₂O₇)₂ (space group Pnnm) and Na₂Cs₆Cl₂(UO₂)₃(Ge₂O₇)₂ ( P2₁/ c) can be constructed from the same SBUs [UGe₄] pentamers. Thermal stability of salt-inclusions was studied by TG and PXRD analysis. 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 concept of SBUs combined with the cutting and gluing strategy was applied to understand and analyze the distinct 8-, 10-, 12-, and 14- membered channels for the uranyl germanate family. The charge density of all known 3D U-Si/Ge-O frameworks has been investigated, which shows strong correlations with chemical composition of corresponding phases. The increase of Si/O (Ge/O) ratios in silicate units results in the decrease of negative charge density. Moreover, the charge density increases with decreasing countercation size within the same Si/O ratio. The correlations can be used to predict inclusion phase formation within U-Si/Ge-O families. Raman spectra of the studied uranyl germanates were measured, and bands were assigned on the basis of structural features.

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. K₁₄(UO₂)₃Si₁₀O₃₀ crystallizes in the P2₁/ c space group and contains open-branched sechser (six) single silicate chains, whereas K₂(UO₂)Si₂O₆ crystallizes in the C2/ c space group and is built of unbranched achter (eight) silicate chains. The crystals of K₁₄(UO₂)₃Si₁₀O₃₀ and K₂(UO₂)Si₂O₆ are related by increasing U/Si molar ratios, and both structures contain the same secondary building units (SBUs), [USi₆] heptamers. The triangle diagram for all known A⁺-UO₂²⁺-SiO₄^4- phases demonstrates the high polymerization level of silicate groups in the system, which was compared with the family of A⁺-UO₂²⁺-BO₃^3-/BO₄^5- compounds. For both thorium silicates, the transformation of K₂ThSi₂O₇ to K₂ThSi₃O₉ was found to be a factor of the reaction time. K₂ThSi₂O₇ crystallizes in the C2/ c space group and belongs to the Na₂Si^VISi₂O₇ structure type. Its 3D framework consists of diorthosilicate Si₂O₇ group and ThO₆ octahedra. Noncentrosymmetric K₂ThSi₃O₉ crystallizes in the hexagonal P6₃ space group and adopts mineral wadeite-type structure based upon triorthosilicate Si₃O₉ rings and ThO₆ 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.

Influence of extreme conditions on the formation and structures of caesium uranium(VI) arsenates

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.