Uranyl Borates Research Articles

Alkali Uranyl Borates: Bond Length, Equatorial Coordination and 5f States

Three uranyl borates, UO2B2O4, LiUO2BO3 and NaUO2BO3, have been prepared by solid state syntheses. The influence of the crystallographic structure on the splitting of the empty 5f and 6d states have been probed using High Energy Resolved Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) at the uranium M4-edge and L3-edge respectively. We demonstrate that the 5f splitting is increased by the decrease of the uranyl U-Oax distance, which in turn correlates with an increased bond covalency. This is correlated to the equatorial coordination change of the uranium. The role of the alkalis as charge compensating the axial oxygen of the uranyl is discussed.

Crystal Growth of Alkali Uranyl Borates from Molten Salt Fluxes: Characterization and Ion Exchange Behavior of A2(UO2)B2O5 (A = Cs, Rb, K).

A new family of layered alkali uranyl borates, A2(UO2)B2O5 (A = Cs, Rb, K), was synthesized as high quality single crystals via high temperature flux growth methods. At room temperature, the compounds are structurally closely related although they crystallize in different monoclinic space groups, specifically P21/c (Cs), C2/m (Rb), and C2/c (K). At a low temperature (100 K), Cs2(UO2)B2O5 becomes isostructural with K2(UO2)B2O5 as the result of a reversible structure transition by Cs2(UO2)B2O5. The title phases represent the first examples of uranyl borates resulting from high temperature flux growth utilizing alkali halide fluxes. The synthesis, structures, and thermal, optical, and ion exchange properties are reported, and modeling of the atomic structure and disorder of the ion exchanged phases is discussed.

Two‐Dimensional Uranyl Borates: From Conventional to Extreme Synthetic Conditions

A systematic investigation of uranyl borates under different synthetic conditions resulted in five new 2D compounds, namely, (H3O)[(UO2)(BO3)], Li[(UO2)(BO3)]·(H2O), α‐K4[(UO2)5(BO3)2O4], β‐K4[(UO2)5(BO3)2O4] and K2.5[(UO2)5(BO3)2O2.5(OH)1.5]·(H2O)2.5. (H3O)[(UO2)(BO3)] and Li[(UO2)(BO3)]·(H2O) were obtained from hydrothermal reactions at 220 °C using the same mineralizer. Both materials possess the uranophane sheet topology with different symmetry of the unit cells. In the structure of (H3O)[(UO2)(BO3)] and Li[(UO2)(BO3)]·(H2O), UO7 pentagonal bipyramids share edges and vertexes with four BO3 planar triangles. α‐K4[(UO2)5(BO3)2O4] and β‐K4[(UO2)5(BO3)2O4] are polytypes. α‐K4[(UO2)5(BO3)2O4] was synthesized from a high‐temperature solid‐state reaction under ambient pressure; whereas β‐K4[(UO2)5(BO3)2O4] was obtained from a high‐temperature/high‐pressure (HT/HP) reaction. Both structures have an identical anion topology, but β‐K4[(UO2)5(BO3)2O4] crystallizes in space group with higher symmetry. In K2.5[(UO2)5(BO3)2O2.5(OH)1.5]·(H2O)2.5, which was obtained from a hydrothermal reaction, UO7 polyhedra share vertexes and edges with two independent BO3 triangles, forming the most complex uranyl borate layers among all five compounds. The different synthetic routes, novel topologies, thermal behavior and Raman spectra are discussed.

Synthesis and Study of the First Zeolitic Uranium Borate

A complex, three-dimensional, open-framework, lead uranyl borate, (H2O)Pb3(UO2)3B14O27, denoted as LUBO, was synthesized via a hydrothermal method. LUBO crystallizes in the hexagonal space group P63/m and exhibits a zeolite-like anionic borate framework (B14O27)12-. The main structural unit of the framework is a tubule consisting of six-membered B6O18 rings. Each ring is connected to the successive one by three diborate groups, and these tubules propagate along the c axis. The tubules possess six-membered ring (MR) windows in the axial direction and 8-MR windows on its sides. Interconnection of the parallel tubules, which consist exclusively of BO4 tetrahedra, is provided by triangular BO3 fragments perpendicular to the axis of the tubules. The framework has large pores as well as channels with 8-MR windows extending along the [100], [010], and [110] directions that are consistent with the overall hexagonal symmetry of the structure. The lead cations occupy 8-MR windows and form [Pb3(H2O)] groups with att...

Divergent Structural Chemistry of Uranyl Borates Obtained from Solid State and Hydrothermal Conditions

A series of novel uranyl borates, K4Sr4[(UO2)13(B2O5)2(BO3)2O12], A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs), and Rb3[(UO2)3(BO3)2O(OH)](H2O), were synthesized using conventional conditions. Among them, K4Sr4[(UO2)13(B2O5)2(BO3)2O12] and A6[(UO2)12(BO3)8O3](H2O)6 were obtained through a high-temperature solid-state reaction method, whereas Rb3[(UO2)3(BO3)2O(OH)](H2O) was synthesized via a hydrothermal reaction. All compounds adopt novel two-dimensional (2D) layered structures in which basic building units (BBUs) consist of corner- or edge-sharing UOx (x = 6, 7 and 8) polyhedra linked with planar BO3 triangles or B2O5 dimers. K4Sr4[(UO2)13(B2O5)2(BO3)2O12] is the first mixed alkali–alkaline earth metal uranyl borate. This compound has the most complex 2D anion topology observed thus far in 2D uranyl borates. The fundamental building block (FBB) in this structure, [(UO2)13(B2O5)2(BO3)2O12]12–, consists of 3 UO8 hexagonal bipyramids and 10 UO7 pentagonal bipyramids connected with 2 BO3 triangles and 2 B2O5 di...

Influence of Synthetic Conditions on Chemistry and Structural Properties of Alkaline Earth Uranyl Borates

Four novel alkaline earth metal uranyl borates, namely, A[(UO2)5(BO3)2O2(OH)2]·5H2O (SrUBO-1, BaUBO-1) and A[(UO2)2(B2O5)O] (SrUBO-2, BaUBO-2) (A = Sr, Ba), have been prepared; SrUBO-1, BaUBO-1, and BaUBO-2 were synthesized using hydrothermal methods, whereas SrUBO-2 was prepared by a traditional high-temperature solid-state method. The compounds SrUBO-1 and BaUBO-1 were found to be iso-structural and crystallized in the centrosymmetric group C2/c. Their structure features two-dimensional anionic layers {[(UO2)5(BO3)2O2(OH)2]2–}n that are composed of [(UO2)5O2(OH)2]4+ clusters and [BO3]3– polyanions. From a topological point of view, this two-dimensional anionic layer can be described as a new 4-nodal net topological type with a point symbol of {35.45}2{36.46.53}3{37.47.57}2. SrUBO-2 and BaUBO-2 are iso-structural and crystallized in space group C2/m. Their structures are based on a three-dimensional framework {[(UO2)2(B2O5)O]2–}n. Within the three-dimensional framework, two-dimensional U(2)–U(1) = U(3) l...

Potassium uranyl borate 3D framework compound resulted from temperature directed hydroborate condensation: structure, spectroscopy, and dissolution studies.

The equatorial coordination nature of the uranyl unit has resulted in only three uranyl borate 3D framework compounds reported so far formed from boric acid flux reactions conducted at 190 °C while all others are 2D layers. Here in this work, by increasing the reaction temperature to 250 °C, a new potassium uranyl borate K[(UO2)B6O10(OH)] (KUBO-4) framework compound is synthesized that shares the same layer topology with the previously reported 2D layered KUBO-1. The 3D structure of KUBO-4 is achieved by interlayer hydroborate condensation. The KUBO-4 was further characterized with single crystal XRD, SHG and fluorescence spectra, and TG/DSC measurements. A deep understanding regarding the dissolution behaviours of uranyl borate is achieved via solubility studies of the KUBO-1 and KUBO-4 performed using a combination of ICP-MS, powder XRD, and fluorescence spectroscopy techniques. The results confirm the lack of stability of borates in aqueous solutions with the presence of coordinating ligands in the environment regardless of the structure types.

Microbial dissolution and reduction of uranyl crystals by Shewanella oneidensis MR-1

Dissimilatory metal-reducing bacteria (DMRB) can harvest energy for growth and activities by respiring metals, but it is so far unknown whether DMRB can acquire crystalline-phase actinides. In the present study, we used Shewanella oneidensis MR-1 to investigate microbially-mediated dissolution and reduction of U(VI) in three uranyl(VI) borate and boronate crystals (i.e., UO2(CH3BO2)(H2O) (UCBO); UO2B2O4 (UBO); and Na[(UO2)B6O10(OH)]·2H2O (NaUBO)). Comparison of the dissolved U(VI) concentrations between samples with and without bacteria indicates that MR-1 facilitated dissolution of UCBO and UBO. Based on the assumption that only dissolved U(VI) was reduced, U(VI) reduction was substantially underestimated for UCBO and NaUBO, indicating that MR-1 directly reduced crystalline U(VI) in these two compounds. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis implied that interactions occurred between microbial ligands and the residual particles of uranyl compounds. We found that S. oneidensis MR-1 can mediate the dissolution and reduction of crystalline U(VI) through facilitated dissolution and consequent reduction of crystals or direct reduction of U(VI) in crystals. These results help evaluate the environmental fate of solid-phase U(VI), critical for predicting U transport and remediating U-contaminated sites.

Further evidence for the stabilization of U(V) within a tetraoxo core.

Two complex layered uranyl borates, K10[(UO2)16(B2O5)2(BO3)6O8]·7H2O (1) and K13[(UO2)19(UO4)(B2O5)2(BO3)6(OH)2O5]·H2O (2), were isolated from supercritical water reactions. Within these compounds, borate exists only as BO3 units and is found as either isolated BO3 triangles or B2O5 dimers, the latter being formed from corner sharing of two BO3 units. These anions, along with oxide and hydroxide, bridge between uranyl centers to create the complex layers in these compounds. U(VI) cations are found within uranyl, UO2(2+) units, that are bound by four or five oxygen atoms to create tetragonal and pentagonal bipyramidal environments. The most striking feature in this system is found in 2, where a [UO4(OH)2] unit exists that contains U(V) within a tetraoxo core with trans hydroxide anions; therefore, this compound is a mixed-valent U(VI)/U(V) borate. The presence of a 5f(1) uranium site within 2 leads to unusual optical properties.

Dissolution of uranyl and plutonyl borates: Influences of crystalline structures and aqueous ligands

Novel crystalline actinide materials can be used to immobilize uranium and transuranium elements at radioactive waste disposal sites. However, the stability and dissolution of such phases are still poorly understood. In this study, we investigated the dissolution kinetics of two uranyl borates and one plutonyl borate, UO2B2O4 (UBO), Na[(UO2)B6O10(OH)]·2H2O (NaUBO), and PuO2[B8O11(OH)4] (PuBO), with three different ligands, i.e., bicarbonate, phosphate, and natural humic acid (HA). These actinide borates were chosen because they possess different topologies that may influence dissolution. In the presence of bicarbonate, the apparent steady-state concentration of dissolved U(VI) and surface area-normalized dissolution rates were much higher for UBO compared to NaUBO. Uranyl phosphate, most likely in the form of (UO2)3(PO4)2(H2O)4, was precipitated on the surface of residual particles of UBO and NaUBO with phosphate. The dissolution rate of PuBO in NaNO3 solution was much higher than UBO or NaUBO. This study revealed that actinide borate crystal dissolution was substantially influenced by crystalline structures of actinide borates and aqueous ligands.

High Structural Complexity of Potassium Uranyl Borates Derived from High-Temperature/High-Pressure Reactions

Three new potassium uranyl borates, K12[(UO2)19(UO4)(B2O5)2(BO3)6(BO2OH)O10] ·nH2O (TPKBUO-1), K4[(UO2)5(BO3)2O4]·H2O (TPKBUO-2), and K15[(UO2)18(BO3)7O15] (TPKBUO-3), were synthesized under high-temperature/high-pressure conditions. In all three compounds, the U/B ratio exceeds 1. Boron exhibits BO3 coordination only, which is different from other uranyl borates prepared at room temperature or under mild hydrothermal conditions. A rare uranium(VI) tetraoxide core UO4O2, which is coordinated by two BO3 groups, is observed in the structure of TPKBUO-1. Both structures of TPKBUO-1 and TPKBUO-3 contain three different coordination environments of uranium, namely, UO4O2, UO2O4, and UO2O5 and UO2O4, UO2O5, and UO2O6 bipyramids in TPKBUO-1 and TPKBUO-3, respectively.

ChemInform Abstract: Recent Progress in Actinide Borate Chemistry

AbstractReview: 85 refs.

Incorporation of iodate into uranyl borates and its implication for the immobilization of 129I in nuclear waste repositories

Abstract A series of uranyl borates, UO2B2O4, Na[(UO2)B6O10(OH)]3· 2H2O, and K2[(UO2)2B12O19(OH)4]·0.3H2O, with different topologies have been prepared and tested for the incorporation of iodate, one of the chemical forms that 129I should have in nuclear waste. Laser ablation ICP-MS was used to determine the iodine content in single crystals of the three different uranyl borates. Iodine levels can be greater than 3200 ppm in crystals synthesized in the presence of iodate. The intercalation of iodic acid is one possible mechanism for iodate uptake in these solids.

Recent progress in actinide borate chemistry

The use of molten boric acid as a reactive flux for synthesizing actinide borates has been developed in the past two years providing access to a remarkable array of exotic materials with both unusual structures and unprecedented properties. [ThB(5)O(6)(OH)(6)][BO(OH)(2)]·2.5H(2)O possesses a cationic supertetrahedral structure and displays remarkable anion exchange properties with high selectivity for TcO(4)(-). Uranyl borates form noncentrosymmetric structures with extraordinarily rich topological relationships. Neptunium borates are often mixed-valent and yield rare examples of compounds with one metal in three different oxidation states. Plutonium borates display new coordination chemistry for trivalent actinides. Finally, americium borates show a dramatic departure from plutonium borates, and there are scant examples of families of actinides compounds that extend past plutonium to examine the bonding of later actinides. There are several grand challenges that this work addresses. The foremost of these challenges is the development of structure-property relationships in transuranium materials. A deep understanding of the materials chemistry of actinides will likely lead to the development of advanced waste forms for radionuclides present in nuclear waste that prevent their transport in the environment. This work may have also uncovered the solubility-limiting phases of actinides in some repositories, and allows for measurements on the stability of these materials.

ChemInform Abstract: Structure—Property Relationships in Lithium, Silver, and Cesium Uranyl Borates.

AbstractThe synthesis of (VII) yields a mixture of prismatic crystals of α‐(VII), and tablet‐like β‐(VII).

Structure−Property Relationships in Lithium, Silver, and Cesium Uranyl Borates

Four new uranyl borates, Li[UO{sub 2}B{sub 5}O{sub 9}]·H{sub 2}O (LiUBO-1), Ag[(UO{sub 2})B{sub 5}O{sub 8}(OH){sub 2}] (AgUBO-1), α-Cs[(UO{sub 2}){sub 2}B{sub 11}O{sub 16}(OH){sub 6}] (CsUBO-1), and β-Cs[(UO{sub 2}){sub 2}B{sub 11}O{sub 16}(OH){sub 6}] (CsUBO-2) were synthesized via the reaction of uranyl nitrate with a large excess of molten boric acid in the presence of lithium, silver, or cesium nitrate. These compounds share a common structural motif consisting of a linear uranyl, UO{sub 2}{sup 2+}, cation surrounded by BO{sub 3} triangles and BO{sub 4} tetrahedra to create an UO{sub 8} hexagonal bipyramidal environment around uranium. The borate anions bridge between uranyl units to create sheets. Additional BO{sub 3} triangles extend from the polyborate layers, and are directed approximately perpendicular to the sheets. In Li[(UO{sub 2})B{sub 5}O{sub 9}]·H{sub 2}O, the additional BO{sub 3} triangles connect these sheets together to form a three-dimensional framework structure. Li[UO{sub 2})B{sub 5}O{sub 9}]·H{sub 2}O and β-Cs[(UO{sub 2}){sub 2}B{sub 11}O{sub 16}(OH){sub 6}] adopt noncentrosymmetric structures, while Ag[(UO{sub 2})B{sub 5}O{sub 8}(OH){sub 2}] and α-Cs[(UO{sub 2}){sub 2}B{sub 11}O{sub 16}(OH){sub 6}] are centrosymmetric. Li[(UO{sub 2})B{sub 5}O{sub 9}]·H{sub 2}O, which can be obtained as pure phase, displays second-harmonic generation of 532 nm light from 1064 nm light. Topological relationships of all actinyl borates are developed.

ChemInform Abstract: Polarity and Chirality in Uranyl Borates: Insights into Understanding the Vitrification of Nuclear Waste and the Development of Nonlinear Optical Materials.

AbstractThe compounds (IV—VII) and (IX—XII) are synthesized with various Na(Tl):U:B molar ratios using H3BO3 as a reactive flux, and their structures are determined by single crystal XRD.

Crystal Chemistry of the Potassium and Rubidium Uranyl Borate Families Derived from Boric Acid Fluxes

The reaction of uranyl nitrate with a large excess of molten boric acid in the presence of potassium or rubidium nitrate results in the formation of three new potassium uranyl borates, K(2)[(UO(2))(2)B(12)O(19)(OH)(4)].0.3H(2)O (KUBO-1), K[(UO(2))(2)B(10)O(15)(OH)(5)] (KUBO-2), and K[(UO(2))(2)B(10)O(16)(OH)(3)].0.7H(2)O (KUBO-3) and two new rubidium uranyl borates Rb(2)[(UO(2))(2)B(13)O(20)(OH)(5)] (RbUBO-1) and Rb[(UO(2))(2)B(10)O(16)(OH)(3)].0.7H(2)O (RbUBO-2). The latter is isotypic with KUBO-3. These compounds share a common structural motif consisting of a linear uranyl, UO(2)(2+), cation surrounded by BO(3) triangles and BO(4) tetrahedra to create an UO(8) hexagonal bipyramidal environment around uranium. The borate anions bridge between uranyl units to create sheets. Additional BO(3) triangles extend from the polyborate layers and are directed approximately perpendicular to the sheets. All of these compounds adopt layered structures. With the exception of KUBO-1, the structures are all centrosymmetric. All of these compounds fluoresce when irradiated with long-wavelength UV light. The fluorescence spectrum yields well-defined vibronically coupled charge-transfer features.

ChemInform Abstract: How Are Centrosymmetric and Noncentrosymmetric Structures Achieved in Uranyl Borates?

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

How are Centrosymmetric and Noncentrosymmetric Structures Achieved in Uranyl Borates?

Four uranyl borates, UO(2)B(2)O(4) (UBO-1), alpha-(UO(2))(2)[B(9)O(14)(OH)(4)] (UBO-2), beta-(UO(2))(2)[B(9)O(14)(OH)(4)] (UBO-3), and (UO(2))(2)[B(13)O(20)(OH)(3)].1.25H(2)O (UBO-4), have been prepared from boric acid fluxes at 190 degrees C. UBO-3 and UBO-4 are centrosymmetric, whereas UBO-1 and UBO-2 are noncentrosymmetric (chiral and polar). These uranyl borates possess layered structures constructed from UO(8) hexagonal bipyramids, BO(3) triangles, and BO(4) tetrahedra. In the case of UBO-4, clusters of BO(3) triangles link the layers together to form open slabs with a thickness of almost 2 nm. The ability of uranyl borates to use very similar layers to yield both centrosymmetric and noncentrosymmetric layers is detailed in this work.