Flux versus Hydrothermal Growth: Polymorphism of A2(UO2)Si2O6 (A = Rb, Cs)

Framework Uranyl Silicates: Crystal Chemistry and a New Route for the Synthesis

To date, uranyl silicates are mostly represented by minerals in nature. However, their synthetic counterparts can be used as ion exchange materials. A new approach for the synthesis of framework uranyl silicates is reported. The new compounds Rb₂[(UO₂)₂(Si₈O₁₉)](H₂O)_2.5 (1), (K,Rb)₂[(UO₂)(Si₁₀O₂₂)] (2), [Rb₃Cl][(UO₂)(Si₄O₁₀)] (3) and [Cs₃Cl][(UO₂)(Si₄O₁₀)] (4) were prepared at harsh conditions in "activated" silica tubes at 900 °C. The activation of silica was performed using 40% hydrofluoric acid and lead oxide. Crystal structures of new uranyl silicates were solved by direct methods and refined: 1 is orthorhombic, Cmce, a = 14.5795(2) Å, b = 14.2083(2) Å, c = 23.1412(4) Å, V = 4793.70(13) ų, R1 = 0.023; 2 is monoclinic, C2/m, a = 23.0027(8) Å, b = 8.0983(3) Å, c = 11.9736(4) Å, β = 90.372(3) °, V = 2230.43(14) ų, R1 = 0.034; 3 is orthorhombic, Imma, a = 15.2712(12) Å, b = 7.9647(8) Å, c = 12.4607(9) Å, V = 1515.6(2) ų, R1 = 0.035, 4 is orthorhombic, Imma, a = 15.4148(8) Å, b = 7.9229(4) Å, c = 13.0214(7) Å, V = 1590.30(14) ų, R1 = 0.020. Their framework crystal structures contain channels up to 11.62 × 10.54 Å filled by various alkali metals.

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.

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.

Flux crystal growth of uranium(v) containing oxyfluoride perovskites

Crystals of three new uranium(v) containing oxyfluorides were grown out of an alkali fluoride flux and adopt a perovskite-type structure and are examined by SXRD, PXRD, XANES, XPS, EDS, magnetic susceptibility measurements, DFT calculations, and UV-vis spectroscopy.

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.

PbLnB7O13 (Ln = Tb or Eu): a new type of layered polyborate with multi-colour light emission properties

This work demonstrates two new polyborates PbLnB₇O₁₃ (Ln = Tb or Eu) with layer structures. Under near-UV light excitation, their solid solutions, PbTb_1−xEu_xB₇O₁₃ (x = 0–1), show a continuously changing emission color from green to red as the x value changes.