U(V) stabilization via aliovalent incorporation of Ln(III) into oxo-salt framework.

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).

Understanding of the structural chemistry in the uranium oxo-tellurium system under HT/HP conditions.

The study of phase formation in the U-Te-O systems with mono and divalent cations under high-temperature high-pressure (HT/HP) conditions has resulted in four new inorganic compounds: K2 [(UO2) (Te2O7)], Mg [(UO2) (TeO3)2], Sr [(UO2) (TeO3)2] and Sr [(UO2) (TeO5)]. Tellurium occurs as TeIV, TeV, and TeVI in these phases which demonstrate the high chemical flexibility of the system. Uranium VI) adopts a variety of coordinations, namely, UO6 in K2 [(UO2) (Te2O7), UO7 in Mg [(UO2) (TeO3)2] and Sr [(UO2) (TeO3)2], and UO8 in Sr [(UO2) (TeO5)]. The structure of K2 [(UO2) (Te2O7)] is featured with one dimensional (1D) [Te2O7]4- chains along the c-axis. The Te2O7 chains are further linked by UO6 polyhedra, forming the 3D [(UO2) (Te2O7)]2- anionic frameworks. In Mg [(UO2) (TeO3)2], TeO4 disphenoids share common corners with each other resulting in infinite 1D chains of [(TeO3)2]4- propagating along the a-axis. These chains link the uranyl bipyramids by edge sharing along two edges of the disphenoids, resulting in the 2D layered structure of [(UO2) (Te2O6)]2-. The structure of Sr [(UO2) (TeO3)2] is based on 1D chains of [(UO2) (TeO3)2]∞ 2- propagating into the c-axis. These chains are formed by edge-sharing uranyl bipyramids which are additionally fused together by two TeO4 disphenoids, which also share two edges. The 3D framework structure of Sr [(UO2) (TeO5)] is composed of 1D [TeO5]4- chains sharing edges with UO7 bipyramids. Three tunnels based on 6-Membered rings (MRs) are propagating along [001] [010] and [100] directions. The HT/HP synthetic conditions for the preparation of single crystalline samples and their structural aspects are discussed in this work.

Achieving and Stabilizing Uranyl Bending via Physical Pressure.

Applying physical pressure in the uranyl-sulfate system has resulted in the formation of the first purely inorganic uranyl oxo-salt phase with a considerable uranyl bend: Na4[(UO2)(SO4)3]. In addition to a strong bend of the typically almost linear O═U═O, the typically equatorial plane is broken up by two out-of-plane oxygen positions. Computational investigations show the origin of the bending to lie in the applied physical pressure and not in the electronic influence or steric hindrance. The increase in pressure onto the system has been shown to increase uranyl bending. Furthermore, the phase formation is compared with a reference phase of a similar structure without uranyl bending, and a transition pressure of 2.5 GPa is predicted, which is well in agreement with the experimental results.

Insights into the Structural Chemistry of Anhydrous and Hydrous Hexavalent Uranium and Neptunium Dinitrato, Trinitrato, and Tetranitrato Complexes.

A systematic investigation is presented which examines the structural chemistry of anhydrous and hydrous ternary hexavalent uranium and neptunium dinitrato, trinitrato, and tetranitrato complexes. Using slow evaporation methods under acidic conditions the uranium and neptunium nitrate complexes γ-K[UO2(NO3)3], K2[UO2-cis-(NO3)4], [NpO2(NO3)2(H2O)2]·4H2O, and Cs[NpO2(NO3)3] have been synthesized and their structures refined using single-crystal X-ray diffraction data. γ-K[UO2(NO3)3] adopts an orthorhombic structure in space group Pbca consisting of antiparallel aligned [UO2(NO3)3]- moieties. K2[UO2-cis-(NO3)4] adopts a monoclinic structure in space group P21/c consisting of [UO2(NO3)4]2- moieties with two monodentate and two bidentate nitrate ligands that are arranged in a cis configuration about the uranyl, UO22+, center. Previous investigations have only identified trans variants of this monoclinic structure, and this is the first report of the cis form and also the occurrence of geometric isomerism in uranyl nitrates. [NpO2(NO3)2(H2O)2]·4H2O adopts an orthorhombic structure in space group Cmc21 consisting of parallel aligned [NpO2(NO3)2(H2O)2] moieties that are in a trans configuration with respect to the bidentate nitrate ligands. Cs[NpO2(NO3)3] adopts a hexagonal structure in space group R3c consisting of parallel aligned [NpO2(NO3)3]- moieties. It was found that despite using a Np(V) nitrate solution as the starting reagent, Np(VI) nitrate structures were consistently recovered under acidic conditions. These observations are discussed and rationalized with respect to standard reduction potentials, particularly how redox conditions and acidity affect the oxidation state of Np and subsequent structure formation. The structures uncovered in this investigation are discussed comparatively and systematically in detail with other reported anhydrous and hydrous ternary hexavalent uranium and neptunium dinitrato, trinitrato, and tetranitrato complexes, particularly with respect to how synthesis conditions, including pH and geometric isomerism, affect the structural chemistry.

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 Å2. ß-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 (TOx, 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.

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, ß-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 ß-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.

Unexpected Behavior of Np in Oxo-selenate/Oxo-selenite Systems.

A study of neptunium (Np) chemistry in the complex oxo-selenium system has been performed. Hereby, two sets of precipitation experiments were conducted, investigating the influence of the initial oxidation state of selenium using SeIVO2 and H2SeVIO4 with NpV in alkali nitrate solution, keeping the ratio of Np/Se constant. Surprising results were observed. Five novel neptunium and selenium bearing compounds have been obtained by slow evaporation from aqueous solution. The novel NpIV phase K4-x[Np(SeO3)4-x(HSeO3)x]·(H2O)1.5 (1) crystallizes in green-colored, plate-shaped crystals and was obtained by adding SeO2 and ANO3 to a NpV stock solution. Single-crystal X-ray diffraction reveals one-dimensional chain structures composed of square antiprismatic NpO8 polyhedra linked via four trigonal pyramidal SeO3 and HSeO3 units. Raman spectral analysis supports the presence of both selenite and hydroselenite due to the presence of corresponding modes within the spectra. The addition of selenic acid to a NpV stock solution resulted in the precipitation of elongated rose prisms of K2[(NpO2)2(SeO4)3(H2O)2]·(H2O)1.5 (2), Rb2[(NpO2)2(SeO4)3(H2O)2]·(H2O)2 (3) and K9[(NpO2)9(SeO4)13.5(H2O)6]·(H2O)12 (4) as well as light red plates of Cs2[(NpO2)2(SeO4)3] (5). To our knowledge, this is the first report of NpVI selenates. All four structures show two-dimensional layered structures with alkali cations acting as charge balancing counter cations. Hereby the layers of compounds 2 and 3 are found to be orientational geometric isomers. Distinctly different phenomena are made responsible for the phase formation within these systems. The kinetically driven process of NpV disproportionation led to the formation of the NpIV selenites in the SeIV-based system, whereas the oxidation of NpV by reduction of nitrate in acidic conditions is responsible for the formation of the NpVI selenates in the SeVI system. The influence of air oxygen is also discussed for the latter reaction.

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), ß-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.

Chemical and structural evolution in the Th-SeO3(2-)/SeO4(2-) system: from simple selenites to cluster-based selenate compounds.

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, ß-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 ß-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 SeO4(2-). The positions of the vibrational modes associated with both Se(IV)O3(2-) and Se(VI)O4(2-) units, respectively, were determined for four compounds, and the Raman spectra of α- and ß-Th(SeO3)2 are compared and discussed in detail.