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

Hydrothermal Synthesis and Structural Investigation of a Crystalline Uranyl Borosilicate

The relevance of multidimensional and porous crystalline materials to nuclear waste remediation and storage applications has motivated exploratory research focused on materials discovery of compounds, such as actinide mixed-oxoanion phases, which exhibit rich structural chemistry. The novel phase K1.8Na1.2[(UO2)BSi4O12] has been synthesized using hydrothermal methods, representing the first example of a uranyl borosilicate. The three-dimensional structure crystallizes in the orthorhombic space group Cmce with lattice parameters a = 15.5471(19) Å, b = 14.3403(17) Å, c = 11.7315(15) Å, and V = 2615.5(6) Å3, and is composed of UO6 octahedra linked by [BSi4O12]5− chains to form a [(UO2)BSi4O12]3− framework. The synthesis method, structure, results of Raman, IR, and X-ray absorption spectroscopy, and thermal stability are discussed.

Chemistry of Metal Silicates and Germanates: The Largest Metal Polygermanate, K11Mn21Ge32O86(OH)9(H2O), with a 76 Å Periodic Lattice

An examination of manganese silicates and germanates revealed unusual structural motifs and extremely different chemistries, with identical hydrothermal reactions forming K₂Mn₂Si₃O₉ versus K₁₁Mn₂₁Ge₃₂O₈₆(OH)₉(H₂O). The germanate is exceptional in both its c-axis length (exceeding 76 Å) and unit cell volume (nearly 18000 ų), the largest known polygermanate structure to our knowledge.

Crystal growth of novel 3D skeleton uranyl germanium complexes: influence of synthetic conditions on crystal structures

Five centrosymmetric uranyl germanate compounds, K₈BrF(UO₂)₃(Ge₂O₇)₂, Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O, Cs₆(UO₂)₂Ge₈O₂₁ and A⁺₂(UO₂)₃(GeO₄)₂ (A⁺ = Rb⁺, Cs⁺), were synthesized in this work. K₈BrF(UO₂)₃(Ge₂O₇)₂ and Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O were obtained under mixed KF-KBr flux and hydrothermal conditions, respectively. Both structures crystallized in the triclinic P1[combining macron] space group and have similar anionic frameworks featuring novel hexagon shaped 12-membered channels. The condensation of two different types of SBU [UGe₄] pentamers (A) and (A2) results in the formation of K₈BrF(UO₂)₃(Ge₂O₇)₂ and Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O frameworks. Cs₆(UO₂)₂Ge₈O₂₁ was obtained from a CsF-CsCl high temperature flux, and it also crystallized in the centrosymmetric triclinic P1[combining macron] space group. The structure of Cs₆(UO₂)₂Ge₈O₂₁ has a novel oxo-germanate layer composed of germanate tetrahedra and trigonal bipyramids. Two new SBU types, (4²·5²-A2) and (5⁴-A2) [UGe₄] pentamers, were found in the structure of Cs₆(UO₂)₂Ge₈O₂₁. A⁺₂(UO₂)₃(GeO₄)₂ (A⁺ = Rb⁺, Cs⁺) were synthesized by a high temperature/high pressure (HT/HP) technique, and both structures with oval-shaped 12-membered channels crystallized in the centrosymmetric orthorhombic Pnma space group. The extreme conditions led to the formation of [U₂Ge₂] tetramers (E), which consist of 7-coordinated U and 5-coordinated Ge. Different synthetic methods of uranyl germanate compounds resulted in a distinct coordination environment of the uranyl cations and a variety of U[double bond, length as m-dash]O and U-O bond lengths, further affecting the dimensionality and types of uranyl units and SBUs. The Raman and IR spectra of the five new phases were collected and analyzed.

Uranyl oxide hydrate frameworks with lanthanide ions

The first two uranyl oxide hydrate frameworks incorporating lanthanide ions (Ln = Eu³⁺/Gd³⁺) have been synthesized hydrothermally and characterized.

Structural and Spectroscopic Investigation of Novel 2D and 3D Uranium Oxo-Silicates/Germanates and Some Statistical Aspects of Uranyl Coordination in Oxo-Salts

Synthesis, structural and spectroscopic characterization, and topological analysis of five novel uranyl-based silicates and germanates have been performed. The open-framework K₄(UO₂)₂Si₈O₂₀·4H₂O has been synthesized under hydrothermal conditions and is based upon [USi₆] heptamers interconnected via edge-sharing. Its structure is composed of sechser silicate layers with 4-, 8-, and 16-membered rings. The largest 16-membered rings have an average dimension of ∼8.93 × 9.42 Ų. β-K₂(UO₂)Si₄O₁₀ has been obtained by the high-temperature flux growth method. Its 3D framework contains a loop-branched sechser single layer with 4- and 8-membered rings and consists of the same [USi₆] heptamers as observed in K₄(UO₂)₂Si₈O₂₀·4H₂O. Na₆(UO₂)₃(Si₂O₇)₂ has also been synthesized from melted fluxes and represents a 2D layer structure composed by [USi₄] pentamers. Two iso-structural compounds A⁺(UO₂)(HGeO₄)·H₂O (A⁺ = Rb⁺, Cs⁺) were synthesized via the hydrothermal method, and their structures are of the α-uranophane type. The 2D layers consist of [U₂Ge₂] tetramer secondary building units (SBUs). The Raman spectra of all novel phases were collected, and bands were assigned according to the existing oxo-silicate rings and oxo-germanium units. Additionally, we performed a statistical investigation of the local coordination of uranyl ions in all known inorganic structures with different oxo-anions (TO_x, T = B³⁺, Si/Ge⁴⁺, P/As⁵⁺, S/Se/Te⁶⁺, Cr/Mo/W⁶⁺, P/As³⁺, and Se/Te⁴⁺). We found a direct correlation between the ionic potential of the central cations T in oxo-anions in their higher oxidation states and the coordination number of uranyl groups.

Mg[(UO2)2(Ge2O6(OH)2]·(H2O)4.4, a novel compound with mixed germanium coordination: cation disordering and topological features of β-U3O8 type sheets

A novel hydrated magnesium uranyl germanate, Mg[(UO2)2(Ge2O6(OH)2)]·(H2O)4.4, has been synthesized under hydrothermal conditions at 200 °C. The orthorhombic unit-cell parameters are a=10.829(6), b=7.625(4), c=16.888(10) Å, V=1394.5(1) Å3, space group Cmcm, Z=4. The crystal structure is based on β-U3O8-type sheets of corner- and edge-sharing U6+O7 pentagonal bipyramids. The GeO3(OH) tetrahedra and GeO4(OH) trigonal bipyramids are linked to form [Ge2φ8] diortho groups that fill the hexagonal-shaped windows within the sheets. The uranyl germanate layers are connected through Mgφ6 octahedra. The disorder of the [Ge2φ8] diortho groups leads to different local structure types with layered- and framework-like characters. A review of the crystal structures of uranyl minerals and actinide-bearing synthetic compounds based on β-U3O8 topological-type sheets is provided. Structural complexity parameters (I G,total=176.19 bits/unit cell) indicate that the title compound is one of the simplest actinyl compounds among this family.

Mineralogy, Crystallography and Structural Complexity of Natural Uranyl Silicates

Naturally occurring uranyl silicates are common constituents of the oxidized parts (i.e., supergene zone) of various types of uranium deposits. Their abundance reflects the widespread distribution of Si4+ in the Earth’s crust and, therefore, in groundwaters. Up to date, 16 uranyl silicate minerals are known. Noteworthy is that the natural uranyl silicates are not extremely diverse regarding their crystal structures; it is a result of possible concentrations (activity) of Si4+ in aqueous solutions derived from dissolution of primary Si minerals or the composition of late hydrothermal fluids. Therefore, in natural systems, we distinguish in fact among two groups of uranyl silicate minerals: uranophane and weeksite-group. They differ in U:Si ratio (uranophane, 1:1; weeksite, 2:5) and they form under different conditions, reflected in distinctive mineral associations. An overview of crystal-chemistry is provided in this paper, along with the new structure data for few members of the uranophane group. Calculations of the structural complexity parameters for natural uranyl silicates are commented about as well as other groups of uranyl minerals; these calculations are also presented from the point of view of the mineral paragenesis and associations.

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.