Role of Anions and Reaction Conditions in the Preparation of Uranium(VI), Neptunium(VI), and Plutonium(VI) Borates

Flux Synthesis of Alkaline-Earth Lanthanide Borates as Potential Nuclear Waste Forms

Neodymium is typically considered the best surrogate for trivalent americium and can be used to identify Am3+ containing materials that are likely to form. We have explored the alkaline-earth lanthanide borate phase space using alkaline-earth halide/carbonate fluxes. This resulted in the synthesis of new compounds AE5Ln(BO3)4X (AE = Ca, Sr; Ln = Pr, Nd, Eu, Tb; X = Cl, Br) and AE3Ln2(BO3)4 (AE = Sr, Ba; Ln = Pr, Nd) as well as the synthesis of two compounds of Ba8Ln2(BO3)6(B2O5) (Ln = Eu, Tb) crystallizing in a new structure type. Ba8Ln2(BO3)6(B2O5) crystallizes in the space group P21/n with lattice parameters a = 8.6002(3) Å, b = 7.9245(3) Å, c = 17.6697(7) Å, and β = 91.3560(10)° for the Eu analogue, and the structure contains isolated LnO8 polyhedra connected into a framework by BO3 and B2O5 units. The fluorescence emission spectra of AE5Ln(BO3)4X (AE = Ca, Sr; Ln = Eu, Tb; X = Cl, Br) and Ba8Ln2(BO3)6(B2O5) (Ln = Eu, Tb) are reported.

Plutonium and Cerium Perrhenate/Pertechnetate Coordination Polymers and Frameworks

Spent nuclear fuel (SNF) contains transuranic and lanthanide species, which are sometimes recovered and repurposed. One particularly problematic fission product, 99TcO4-, hampers this recovery via coextraction with high valence metals, perhaps by complexation during aqueous reprocessing of SNF. There is limited molecular-level knowledge concerning the coordination chemistry between TcO4- or its well-known surrogate ReO4- and transuranic/lanthanide species. In the current study, we investigated the coordination of ReO4-/TcO4- with plutonium and cerium cations by structural and chemical characterization of a series of isolated extended solids. In this study, Ce represents both trivalent lanthanides and is considered a surrogate for Pu, respectively, in its common trivalent and tetravalent oxidation states. The structural elucidation of the seven isolated crystalline solids revealed that ReO4-/TcO4- directly connects to PuIV, PuVIO22+, CeIII, and CeIV in the terminal and bridging coordination modes, leading to 1-, 2-, and 3-dimensional frameworks. For example, ReO4- coordination to Pu(IV) formed a 1D chain or 2D framework, isostructural with previously isolated Th(IV) compounds. However, PuVIO22+ alternating with ReO4- led to a unique 1D chain, different from the prior-reported U(VI)/Np(VI)-ReO4-/TcO4- structures. Coordination of ReO4-/TcO4- with Ce(III) promotes the assembly of 3D frameworks. Finally, attempted synthesis of a Ce(IV)-ReO4- compound resulted in a 2D framework with a mixed-valence CeIII/IV. The highly acidic reaction conditions supported the reduction of both CeIV and TcVII, challenging isolation of compounds featuring these species. Only one TcO4-containing structure was obtained in this study (CeIII-TcO4 3D framework), vs the six total Ce/Pu-ReO4 compounds. Our three Pu-ReO4 crystal structures are the first reported and translated to atomic-level information about Pu-TcO4 coordination in nuclear fuel reprocessing scenarios, in addition to broadening our knowledge of bonding trends in the early, high-valence actinides.

The crystal chemistry of plutonium(IV) borophosphate

In this work, we report the synthesis and characterization of a plutonium(IV) borophosphate, Pu(H2O)3[B2(OH)(H2O)(PO4)3] (1). The basic building unit of 1 has a B : P ratio of 2 : 3 with an equal number of BO4 and PO4 groups that assemble into 12-membered rings and take on a sheet topology due to presence of hydroxyl groups or a water molecule on one vertex of each BO4 tetrahedron. This unique borophosphate anion topology is not observed in other members of the borophosphate family; it is the plutonium(IV) metal centers, rather than borate or phosphate groups, that link the sheets to form an extended framework. The presence of boron in 1 was confirmed using single crystal X-ray diffraction, electron microprobe analysis, and infrared spectroscopy. Peaks corresponding to the tetrahedral BO45- and tetrahedral PO43- anions were all identified in the fingerprint region (500-1500 cm-1) of the infrared spectrum. Additionally, peaks in the higher wavenumber region corresponded to crystalline water and B-OH vibrations, providing further evidence for the water molecules surrounding plutonium in the structure and the protonation of the BO4 tetrahedron, respectively. This compound represents the first Pu(IV) borophosphate structure and a novel borophosphate anion topology. Furthermore, the long time-frame required for crystallization of 1 and the suspected leaching of boron from the borosilicate vial used during synthesis indicate that 1 could serve as a model for the crystalline materials that are expected to form during the corrosion of vitrified nuclear waste.

Flux Crystal Growth of the Extended Structure Pu(V) Borate Na2(PuO2)(BO3)

As part of our exploration of plutonium-containing materials as potential nuclear waste forms, we report the first extended structure Pu(V) material and the first Pu(V) borate. Crystals of Na₂(PuO₂)(BO₃) were grown out of mixed hydroxide/boric acid flux and found to crystallize in the orthorhombic space group Cmcm with lattice parameters of a = 9.9067(4) Å, b = 6.5909(2) Å, and c = 6.9724(2) Å. Na₂(PuO₂)(BO₃) adopts a layered structure in which layers of PuO₂(BO₃)^2- are separated by sodium cations. Plutonium is found in a pentagonal bipyramidal coordination environment, with axial Pu(V)-O plutonyl bond lengths of 1.876(3) Å and equatorial Pu-O bond lengths ranging from 2.325(5) to 2.467(3) Å. We find that the Pu(V)-O plutonyl bond lengths are approximately 0.1 Å longer than the reported Pu(VI)-O plutonyl bond lengths and shorter by approximately 0.033 Å than the corresponding U(V) uranyl bond lengths. Raman spectroscopy on single crystals was used to determine the PuO₂⁺ plutonyl stretching and the equatorial breathing mode frequencies of the pentagonal bipyramidal coordination environment around plutonium. Density functional theory calculations were used to calculate the Raman spectrum to help identify the Raman bands at 690 and 630 cm^-1 as corresponding to the plutonyl(V) ν1 stretch and the equatorial PuO₅ breathing mode, respectively. UV-vis measurements on single crystals indicate semiconducting behavior with a band gap of ∼2.60 eV.

Progress in solid state and coordination chemistry of actinides in China

In the past decade, the area of solid state chemistry of actinides has witnessed a rapid development in China, based on the significantly increased proportion of the number of actinide containing crystal structures reported by Chinese researchers from only 2% in 2010 to 36% in 2021. In this review article, we comprehensively overview the synthesis, structure, and characterizations of representative actinide solid compounds including oxo-compounds, organometallic compounds, and endohedral metallofullerenes reported by Chinese researchers. In addition, Chinese researchers pioneered several potential applications of actinide solid compounds in terms of adsorption, separation, photoelectric materials, and photo-catalysis, which are also briefly discussed. It is our hope that this contribution not only calls for further development of this area in China, but also arouses new research directions and interests in actinide chemistry and material sciences.

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

A novel polymorph of ThB₂O₅, denoted as β-ThB₂O₅, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, β-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 β polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, β-ThB₂O₅ has smaller molar volume by approximately 12%. Exposing a mixture of the α and β polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being β, 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 β variant at approximately 900 °C via a 1st order mechanism. β-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 β 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, β-ThB₂O₅ could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.

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.

Interaction of Np(v) with borate in alkaline, dilute-to-concentrated, NaCl and MgCl2 solutions

The formation of sparingly soluble ternary Na/Mg–Np(v)–borate(s) solid phases in alkaline, dilute-to-concentrated, NaCl and MgCl₂ solutions is confirmed by a multimethod experimental approach.

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 Se^IVO₂ and H₂Se^VIO₄ with Np^V 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 Np^IV phase K_4-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 Np^V 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 Np^V 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 Np^VI 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 Np^V disproportionation led to the formation of the Np^IV selenites in the Se^IV-based system, whereas the oxidation of Np^V by reduction of nitrate in acidic conditions is responsible for the formation of the Np^VI selenates in the Se^VI system. The influence of air oxygen is also discussed for the latter reaction.

Syntheses, Crystal Structures, and Nonlinear Optical Activity of Cs2Ba[AnO2(C2H5COO)3]4 (An = U, Np, Pu) and Unprecedented Octanuclear Complex Units in KR2(H2O)8[UO2(C2H5COO)3]5 (R = Sr, Ba)

X-ray diffraction was applied to the elucidation of crystal structures of single crystals of Cs₂Ba[AnO₂(C₂H₅COO)₃]₄, where An = U(I), Np(II), Pu(III), and KR₂(H₂O)₈[UO₂(C₂H₅COO)₃]₅, where R = Sr(IV), Ba (polymorphs V-a and V-b). FTIR spectra were analyzed for the uranium-containing crystals I, IV, and V-b. Isostructural cubic crystals I-III are constructed of typical mononuclear anionic complex units [AnO₂(C₂H₅COO)₃]^- and charge-balancing Cs and Ba cations. Features of actinide contraction in the six U-Np-Pu isostructural series known to date are analyzed. In crystal structures of IV and V two typical complexes [UO₂(C₂H₅COO)₃]^- bind with a hydrated Sr or Ba cation to form the rare trinuclear neutral complex unit {R(H₂O)₄[UO₂(C₂H₅COO)₃]₂}, where R = Sr, Ba. Two such trinuclear units and one typical mononuclear unit further bind with a K cation to form the unprecedented octanuclear neutral complex unit K[UO₂(C₂H₅COO)₃]{R(H₂O)₄[UO₂(C₂H₅COO)₃]₂}₂. As the derived polynuclear complexes of uranyl ion with carboxylate ligands in the crystal structures of IV and V are not the first but are rare examples, the equilibrium between mono and polynuclear complex units in aqueous solutions is discussed. The two polymorphic modifications V-a and V-b were studied at 100 K and at room temperature, respectively. Peculiarities of noncovalent interactions in crystal structures of the two polymorphs are revealed using Voronoi-Dirichlet tessellation. The nonlinear optical activity of noncentrosymmetric crystals I was estimated by its ability for second harmonic generation.

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

A potassium uranyl borate K[(UO₂)B₆O₁₀(OH)] (KUBO-4) framework structure was synthesized and characterized with a variety of measurements.

A facile strategy to adjust the density of planar triangle units in lead borate–nitrates

Three novel lead(ii) borate–nitrates were obtained through a facile hydrothermal reaction by adjusting the concentrations of the starting materials.

Isolation of a series of uranium organophosphinates

A series of uranyl organophosphinates including one-dimensional chain or two-dimensional sheet structures has been synthesized using hydroxymethyl phenylphosphinic acid (HMPPA) as the ligand.

Kinetics vs. thermodynamics: a unique crystal transformation from a uranyl peroxo-nanocluster to a nanoclustered uranyl polyborate

Kinetics vs. thermodynamics: a novel method has been developed to make a nano-clustered uranyl polyborate in aqueous solution at room temperature through a unique crystal transformation.

Differentiating between trivalent lanthanides and actinides

The reactions of LnCl(3) with molten boric acid result in the formation of Ln[B(4)O(6)(OH)(2)Cl] (Ln = La-Nd), Ln(4)[B(18)O(25)(OH)(13)Cl(3)] (Ln = Sm, Eu), or Ln[B(6)O(9)(OH)(3)] (Ln = Y, Eu-Lu). The reactions of AnCl(3) (An = Pu, Am, Cm) with molten boric acid under the same conditions yield Pu[B(4)O(6)(OH)(2)Cl] and Pu(2)[B(13)O(19)(OH)(5)Cl(2)(H(2)O)(3)], Am[B(9)O(13)(OH)(4)]·H(2)O, or Cm(2)[B(14)O(20)(OH)(7)(H(2)O)(2)Cl]. These compounds possess three-dimensional network structures where rare earth borate layers are joined together by BO(3) and/or BO(4) groups. There is a shift from 10-coordinate Ln(3+) and An(3+) cations with capped triangular cupola geometries for the early members of both series to 9-coordinate hula-hoop geometries for the later elements. Cm(3+) is anomalous in that it contains both 9- and 10-coordinate metal ions. Despite these materials being synthesized under identical conditions, the two series do not parallel one another. Electronic structure calculations with multireference, CASSCF, and density functional theory (DFT) methods reveal the An 5f orbitals to be localized and predominately uninvolved in bonding. For the Pu(III) borates, a Pu 6p orbital is observed with delocalized electron density on basal oxygen atoms contrasting the Am(III) and Cm(III) borates, where a basal O 2p orbital delocalizes to the An 6d orbital. The electronic structure of the Ce(III) borate is similar to the Pu(III) complexes in that the Ce 4f orbital is localized and noninteracting, but the Ce 5p orbital shows no interaction with the coordinating ligands. Natural bond orbital and natural population analyses at the DFT level illustrate distinctive larger Pu 5f atomic occupancy relative to Am and Cm 5f, as well as unique involvement and occupancy of the An 6d orbitals.

Complex clover cross-sectioned nanotubules exist in the structure of the first uranium borate phosphate

An actinide borate phosphate was prepared via a high temperature solid-state reaction. This phase exhibits unprecedented complex inorganic nanotubular fragments with an external diameter of ~2 × 2 nm. The nanotubular aggregates are based on borate tubes where the exterior of the tubes is decorated with UO(2)(PO(4))(3) moieties to form a complex shape with a cross-section similar to the clover cross.

Interstitial incorporation of plutonium into a low-dimensional potassium borate

The molten boric acid flux reaction of PuBr(3) with KBO(2) at 200 °C results in the formation of large light-yellow crystals of K[B(5)O(7)(OH)(2)]·H(2)O:Pu(4+). Single-crystal X-ray diffraction experiments on the Pu-doped K[B(5)O(7)(OH)(2)]·H(2)O demonstrate two features: (1) K[B(5)O(7)(OH)(2)]·H(2)O:Pu(4+) adopts a one-dimensional borate chain structure with void spaces between the chains. (2) The doping plutonium atoms do not reside on the potassium sites. The latter are not fully occupied. Both laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and energy-dispersive spectrometry analyses indicate that plutonium atoms are uniformly distributed in crystals of K[B(5)O(7)(OH)(2)]·H(2)O with an atomic K:Pu ratio of approximately 65:1 measured by LA-ICP-MS. UV-vis-NIR spectra taken from both freshly made and one day old crystals show that the plutonium present within the crystals is predominantly characterized by Pu(IV). A small amount of Pu(III) is also present initially, but slowly oxidized to Pu(IV) via interaction with oxygen in the air. X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopic measurements confirm that plutonium is mainly present as a form similar to that of a PuO(2) cluster. The combined results suggest that the clusters containing Pu(IV) ions are uniformly distributed in the void spaces between the borate chains.