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).

Ultrafiltration separation of Am(VI)-polyoxometalate from lanthanides.

Partitioning of americium from lanthanides (Ln) present in used nuclear fuel plays a key role in the sustainable development of nuclear energy1-3. This task is extremely challenging because thermodynamically stable Am(III) and Ln(III) ions have nearly identical ionic radii and coordination chemistry. Oxidization of Am(III) to Am(VI) produces AmO22+ ions distinct with Ln(III) ions, which has the potential to facilitate separations in principle. However, the rapid reduction of Am(VI) back to Am(III) by radiolysis products and organic reagents required for the traditional separation protocols including solvent and solid extractions hampers practical redox-based separations. Herein, we report a nanoscale polyoxometalate (POM) cluster with a vacancy site compatible with the selective coordination of hexavalent actinides (238U, 237Np, 242Pu and 243Am) over trivalent lanthanides in nitric acid media. To our knowledge, this cluster is the most stable Am(VI) species in aqueous media observed so far. Ultrafiltration-based separation of nanoscale Am(VI)-POM clusters from hydrated lanthanide ions by commercially available, fine-pored membranes enables the development of a once-through americium/lanthanide separation strategy that is highly efficient and rapid, does not involve any organic components and requires minimal energy input.

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.

Understanding the role of flux, pressure and temperature on polymorphism in ThB2O5.

A novel polymorph of ThB2O5, denoted as ß-ThB2O5, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, ß-ThB2O5 was found to form a three-dimensional (3D) framework structure where thorium atoms are ten-fold oxygen coordinated forming tetra-capped trigonal prisms. The only other known polymorph of ThB2O5, denoted α, synthesised herein using a known borax, B2O3-Na2B4O7, high temperature solid method, was found to transform to the ß polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, ß-ThB2O5 has smaller molar volume by approximately 12%. Exposing a mixture of the α and ß polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being ß, with no α phase material being observed via Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. In situ heating PXRD measurements on α-ThB2O5 from RT to 1030 °C indicated that α-ThB2O5 transforms to the ß variant at approximately 900 °C via a 1st order mechanism. ß-ThB2O5 was found to exist only over a narrow temperature range, decomposing above 1050 °C. Ab initio calculations using density functional theory (DFT) with the Hubbard U parameter indicated, consistent with experimental observations, that ß is both the preferred phase at higher temperatures and high pressures. Interestingly, it was found by switching from B2O3-Na2B4O7 to H3BO3-Li2CO3 flux using consistent high temperature solid state conditions for the synthesis of the α variant, ß-ThB2O5 could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.

Advances and perspectives of actinide chemistry from ex situ high pressure and high temperature chemical studies.

High pressure high temperature (HP/HT) studies of actinide compounds allow the chemistry and bonding of among the most exotic elements in the periodic table to be examined under the conditions often only found in the severest environments of nature. Peering into this realm of physical extremity, chemists have extracted detailed knowledge of the fundamental chemistry of actinide elements and how they contribute to bonding, structure formation and intricate properties in compounds under such conditions. The last decade has resulted in some of the most significant contributions to actinide chemical science and this holds true for ex situ chemical studies of actinides resulting from HP/HT conditions of over 1 GPa and elevated temperature. Often conducted in tandem with ab initio calculations, HP/HT studies of actinides have further helped guide and develop theoretical modelling approaches and uncovered associated difficulties. Accordingly, this perspective article is devoted to reviewing the latest advancements made in actinide HP/HT ex situ chemical studies over the last decade, the state-of-the-art, challenges and discussing potential future directions of the science. The discussion is given with emphasis on thorium and uranium compounds due to the prevalence of their investigation but also highlights some of the latest advancements in high pressure chemical studies of transuranium compounds. The perspective also describes technical aspects involved in HP/HT investigation of actinide compounds.

Incorporation of iodine into uranium oxyhydroxide phases.

Herein, we have synthesised a novel uranium oxyhydroxide (UOH) phase, Rb2K2[(UO2)6O4(OH)6]·(IO3)2, under hydrothermal conditions which intercalates IO3-via a hybrid salt-inclusion and host-guest mechanism. The mechanism is based on favorable intermolecular bonding between disordered Rb+/K+ and IO3- ions and hydroxyl and layer void positions respectively. To examine whether the intercalation may occur ubiquitously for UOH phases, the known UOH mineral phases metaschoepite ([(UO2)8O2(OH)12]·12H2O), compreignacite (K2[(UO2)6O4(OH)6]·7H2O) and also related ß-UO2(OH)2 were synthesised and exposed to aqueous I- and IO3- for 1 month statically at RT and 60 °C in air and the solid analysed using laser ablation inductively coupled plasma mass spectroscopy. Measurements indicate intercalation can occur homogeneously, but the affinity is dependent upon the structure of the UOH phases and temperature, where higher temperatures and when the interlayer space is free of initial moieties are favoured. It was also found that after repeated washing of the UOH samples with DI water the intercalated iodine was retained. UOH phases are known to form during the oxidative corrosion of spent nuclear fuel during an accident scenario in the near field, this work suggests they may help retard the transport of radiolytic iodine into the environment during a long-term release event.

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.

Oxygen Nonstoichiometry and Valence State of Manganese in La1-x Ca x MnO3+δ.

Perovskites of the ABO3 type, such as LaMnO3, can be used as air electrodes in solid oxide fuel cells and electrolyzers. Their properties can be tuned by A- and B-site substitutions. The influence of La substitution by Ca on the oxygen nonstoichiometry has been investigated frequently, but the results depend highly on the synthesis and atmospheric conditions. In this work, a series of La1-x Ca x MnO3+δ (x = 0-0.5) was synthesized using conventional solid-state synthesis under an air atmosphere. The structures of the materials were studied in detail with powder X-ray diffraction. The initial oxygen nonstoichiometries were determined using thermogravimetric reduction. The samples were subsequently analyzed in terms of defect chemistry in dependence of temperature, atmosphere, and Ca content via thermogravimetric analysis. The changes in the manganese charge states were investigated by X-ray absorption near-edge spectroscopy experiments. The influence of intrinsic and extrinsic effects on the Mn-valence state of the differently Ca-substituted samples as calculated from thermogravimetric analysis and as determined directly from X-ray absorption near-edge spectroscopy is presented.

Tilting and Distortion in Rutile-Related Mixed Metal Ternary Uranium Oxides: A Structural, Spectroscopic, and Theoretical Investigation.

A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4 oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of ß-CdUO4, MnUO4, CoUO4, and MgUO4 are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of ß-CdUO4 is best described by space group Cmmm whereas MnUO4, CoUO4, and MgUO4 are described by the lower symmetry Ibmm space group and are isostructural with the previously reported ß-NiUO4 [Murphy et al. Inorg. Chem. 2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A2+ and U6+ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO6 polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO4 oxides results in reduction of the tilt angle. This is demonstrated for MnUO4 where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO4 is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4 family of oxides.

The first actinide polyiodate: a complex multifunctional compound with promising X-ray luminescence properties and proton conductivity.

Herein we report the first example of an actinide polyiodate compound, namely K4[(UO2)2(IO3)6(I4O11)]·(HIO3)4(H2O)6 (UPI-1), which was obtained from slow evaporation of nitric acid with a high I/U ratio. Spectroscopic measurements indicate that UPI-1 possesses X-ray luminescence properties applicable for X-ray scintillation and also exhibits modest protonic conduction.

Extreme condition high temperature and high pressure studies of the K-U-Mo-O system.

Herein the first examples of alkali earth uranyl molybdates synthesised using extreme conditions of high temperature and high pressure (HT/HP) methods, namely K2[UO2(Mo2O7)2], K2[(UO2)2(Mo(vi)4Mo(iv)(OH)2)O16], K3[(UO2)6(OH)2(MoO4)6(MoO3OH)] and K5[(UO2)10MoO5O11OH]·H2O, are described and characterised. K2[UO2(Mo2O7)2] forms a monoclinic 2D layered structure in space group P21/c that consists of interlinking Mo2O7 dimers that link isolated UO22+ moieties forming [UO2(Mo2O7)2]2- layers which are separated by K+ cations. K2[(UO2)2(Mo(vi)4Mo(iv)(OH)2)O16] forms a disordered triclinic 3D framework structure in space group P1[combining macron]. The structure consists of isolated UO22+ moieties connected in a layered configuration via Mo(vi)O6 polyhedra of which the layers are bridged by Mo(iv)O6 polyhedra that are partially positionally disordered by charge balancing K+ and bridging Mo4+ cations. K3[(UO2)6(OH)2(MoO4)6(MoO3OH)] adopts a disordered orthorhombic 3D framework structure in space group Pbcm consisting of small channels and large cavities built upon corner sharing MoO4 and UO22+ moieties that respectively encapsulate ordered and disordered K+ cations. K5[(UO2)10MoO5O11OH]·H2O forms a triclinic 3D framework structure in space group P1[combining macron] consisting of interlinking UO6, UO7 and MoO5 polyhedra which utilise cation-cation interactions between UO22+ moieties to create infinite channels parallel to the [001] direction which contain partially disordered K+ cations and H2O molecules. A combination of single crystal X-ray diffraction, bond valence sums calculations and scanning electron microscopy with energy dispersive X-ray spectroscopic measurements was used to characterise all obtained samples in this investigation. The structures uncovered in this investigation are discussed systematically in detail with other members of the broader A+-U-Mo-O system from the literature where the relationship between the degree of pressure applied and U/Mo ratio used during synthesis on the ability to obtain high dimensional structures via condensation and oligomerization of polyhedra is identified and discussed in detail.

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.

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.

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.

Overstepping Löwenstein's Rule-A Route to Unique Aluminophosphate Frameworks with Three-Dimensional Salt-Inclusion and Ion-Exchange Properties.

The synthesis of four non-Löwenstein uranyl aluminophosphates, [Cs13Cl5][(UO2)3Al2O(PO4)6], Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2], Cs3[Al2O(PO4)3][(UO2)3O2], and Rb3[Al2O(PO4)3][(UO2)3O2], the first uranyl phosphate salt-inclusion material [Cs4Cs4Cl][(UO2)4(PO4)5], and a related structure Cs4[UO2Al2(PO4)4], all prepared by molten flux methods, is reported. All compounds are discussed from the point of view of their structural features favoring, in some cases, ion-exchange properties. Löwenstein's rule, well known in the realm of zeolites, aluminosilicate, and aluminophosphate minerals, describes the tendency of tetrahedra (Al, P, Si, and Ge) linked by an oxygen bridge to be of two different elements resulting in the avoidance of Al-O-Al bonds. Zeolites and related aluminosilicate/aluminophosphate minerals are traditionally formed under relatively mild temperatures, where zeolites are synthesized using the hydrothermal synthetic technique. Few exceptions to Löwenstein's rule are known among aluminophosphates, and four of the five exceptions are synthesized under either high temperature or high pressure methods. For that reason, the high-temperature flux synthesis of four new non-Löwenstein uranyl aluminophosphates realizes a unique synthetic approach to forming the new pyroaluminate-based building block, [Al2O(PO4)6]14-, that can be easily obtained and employed for the construction of new porous structures.

High-Pressure Synthesis, Structural, and Spectroscopic Studies of the Ni-U-O System.

The first comprehensive structural study of the Ni-U-O system is reported. Single crystals of α-NiUO4, ß-NiUO4, and NiU3O10 were synthesized, and their structures were refined-using synchrotron single-crystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO4 adopts an orthorhombic structure in space group Pbcn and is isostructural to CrUO4 containing corrugated two-dimensional (2D) layers of corner-sharing UO6 polyhedra and edge-sharing one-dimensional (1D) zigzag α-PbO2 rutile-like chains of NiO6 polyhedra in the [001] direction. ß-NiUO4 is isostructural to MgUO4 and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO6 and NiO6 polyhedra in the [001] direction as in regular TiO2 rutile. NiU3O10 forms a triclinic structure in space group P1̅ and is isostructural with CuU3O10, where it forms a three-dimensional (3D) framework structure built through a mixture of UO6 and UO7 polyhedra in which the NiO6 polyhedra sit isolated within the framework. X-ray absorption near-edge structure (XANES) measurements, conducted using XANES mapping of single crystals, support the presence of hexavalent uranium in the three structures. The polymorphs of NiUO4 were found to only form under high-pressure and high-temperature conditions (≥4 GPa and 700 °C). It is argued that this is a consequence of the relative size difference between the Ni2+ and U6+ cations, where the Ni2+ cation is effectively too small for the Ibmm structure and too large for the Pbcn structure to form under ambient pressure conditions. This does not appear to be an issue for NiU3O10, which forms under ambient pressure conditions, where NiO6 polyhedra sit isolated within the framework of 3D connected UO6/UO7 polyhedra. Synthesis conditions indicate that ß-NiUO4 is the preferred higher-pressure phase and that the transformation to this occurs irreversibly at a temperature between 950 and 1000 °C, when P = 4 GPa. The routes toward the synthesis of the oxides and the associated structural and spectroscopic results are described with respect to the structural chemistry of the Ni-U-O system, the larger AUO4 family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.

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.

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+-SiO44- phases demonstrates the high polymerization level of silicate groups in the system, which was compared with the family of A+-UO22+-BO33-/BO45- 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 Na2SiVISi2O7 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.

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

Four novel uranyl silicates and germanates with framework structures, K4Na2(UO2)3(Si2O7)2·3H2O, K4Na2(UO2)3(Ge2O7)2·3H2O, H3O(UO2)2(HGe2O7)·2H2O, and Na2(UO2)GeO4, have been synthesized by means of the hydrothermal method. The structures of the title compounds were refined by single-crystal X-ray diffraction and characterized by Raman spectroscopy. We used the method of secondary building units (SBUs) for a crystal chemical analysis of the 3D framework and their topologies. The framework of the K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) series exhibits large 14-membered rings and smaller 8-membered rings which are built upon [UT4] pentamers. The internal size of the largest pores is approximately 12.39 × 3.33 Å2. H3O(UO2)2(HGe2O7)·2H2O is based on 10-membered rings with intermediate sized pores. They are built upon [U2Ge2] tetramers with 7-fold-coordinated U. The internal dimension of the pores in H3O(UO2)2(HGe2O7)·2H2O is smaller compared to the K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) series with ∼5.91 × 5.33 Å2. Its topology is similar to several uranium germanate synthetic phases and silicate minerals, especially α- and ß-uranophane which are constructed from similar building units. A novel 3D framework type of Na2(UO2)GeO4 with 8-membered rings demonstrates the smallest free volume in the family of porous uranium germanates. It crystallizes in tetragonal symmetry and is built upon corner sharing of [UGe4] pentamers. The size of the channels is ∼6.76 × 4.27 Å2. The vibrational bands in Raman spectra were associated with pyro-(Si2O7)6- and -(Ge2O7)6- groups, with the Ge-OH bond and with H3O+ cations, confirming the results of the X-ray crystallographic structural characterization. We systemized existing uranyl silicates and germanates based on their building units and chemical composition. We found a simple structural dependence between synthetic conditions and chemical composition.

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.