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Two uranium oxide hydrate frameworks (UOHFs) with either Dy3+ or Lu3+ ions, Dy1.36(H2O)6[(UO2)10UO13(OH)4] (UOHF‐Dy) or Lu2(H2O)8[(UO2)10UO14(OH)3] (UOHF‐Lu), were synthesized hydrothermally and characterized with a range of structural and spectroscopic techniques. Although SEM‐EDS analysis confirmed the same atomic ratio of ~5.5 for U : Dy and U : Lu, they displayed different crystal morphologies, needles for UOHF‐Dy in the orthorhombic C2221 space group and plates for UOHF‐Lu in the triclinic P‐1 space group. Both frameworks are composed of β‐U3O8 type layers linked by pentagonal bipyramidal uranium polyhedra, with the Dy3+/Lu3+ ions inside the channels. However, the arrangements of Dy3+/Lu3+ ions are different, with disordered Dy3+ ions well aligned at the centers of the channels and single Lu3+ ions well‐separated in a zigzag pattern in the channels. While the characteristic vibrational modes were revealed by Raman spectroscopy, the presence of a pentavalent uranium center in UOHF‐Lu was confirmed with diffuse reflectance spectroscopy. The formation of two types of UOHFs with lanthanide ions, high or low symmetry, and the structure trend were discussed regards to synthesis conditions and lanthanide ionic radius. This work highlights the complex chemistry driving the formation of UOHFs with lanthanide ions and has implications to the spent nuclear fuel under geological disposal. [ABSTRACT FROM AUTHOR]

Alkaline earth metal ions play an important role in the formation of secondary uranium minerals due to their abundance in the Earth's crust. Although uranium oxide hydrate (UOH) minerals and synthetic phases with calcium, strontium and barium ions have been investigated, their counterparts with magnesium ions are much less studied. In this work, synthetic UOH materials with magnesium ions have been investigated with three new compounds being synthesised and characterised. Compound Mg2(H3O)2(H2O)6[(UO2)3O4(OH)]2 (U-Mg1 with a U : Mg ratio of 3 : 1) crystallises in the monoclinic P21/c space group having a layered crystal structure, constructed by β-U3O8 layers with 6-fold coordinated Mg2+ ions as interlayer cations. Compound Na2Mg(H2O)4[(UO2)3O3(OH)2]2 (U-Mg2p with U : Mg : Na ratios of 6 : 1 : 2) crystallises in the triclinic P1¯ space group having a layered structure, constructed by a unique type of uranium oxide hydroxide layer containing both α-U3O8 and β-U3O8 features, with alternating layers of 6-fold coordinated Mg2+ and 6-/8-fold coordinated Na+ interlayer cations. Compound Na2Mg(H2O)4[(UO2)4O3(OH)4]2 (U-Mg2n with U : Mg : Na ratios of 8 : 1 : 2) crystallises in the triclinic P1¯ space group having a corrugated layer structure, constructed by a unique type of uranium oxide hydroxide layer with mixed 6-fold coordinated Mg2+ and 7-fold coordinated Na+ interlayer cations. The structural diversity in the UOH-Mg system was achieved by adjusting the solution pH using NaOH, highlighting the importance of solution pH control and the additional Na+ ions in the formation of UOH phases. The extra structural flexibility offered by the Na+ ions emphasizes the opportunity for synthesising UOHs with dual-cations to further improve our understanding of the alteration products of spent nuclear fuel under geological disposal. [ABSTRACT FROM AUTHOR]

Although barium(II) ions play an important role in the natural weathering of the mineral uraninite (UO2+x) and are expected to therefore do the same for spent nuclear fuel in geological disposal, the detailed structural understanding of this role is rather limited. In this work, we report the synthesis and characterisation of two new uranium oxide hydrate (UOH) phases with barium(II) ions, a layer-structured UOH-Ba1 constructed with undulating uranyl oxide hydroxide layers incorporating hydrated interlayer barium(II) ions; and UOH-Ba2 forming a complex three-dimensional structure by linking six different types of uranium polyhedrons with barium(II) ions located in the narrow channels. Both crystal structures were revealed using synchrotron single crystal X-ray diffraction and confirmed with powder X-ray or electron diffractions. While vibrational modes were elucidated by Raman, electronic structures and the presence of pentavalent uranium were investigated using diffuse reflectance spectroscopy. This work has provided insights into the complexity of uranium crystal chemistry in the presence of barium(II) ions and has implications to both uranium geochemistry in terms of the natural weathering of uraninite and the disposal of spent nuclear fuel in the underground repository. [ABSTRACT FROM AUTHOR]

We report the synthesis of two new dual-cation uranium oxide hydrate (UOH) materials, containing both Cd2+ and K+ ions, along with their characterisation by means of single-crystal X-ray diffraction and a range of other structural and spectroscopic techniques. The materials were found to differ in structures, topology and uranium to cation ratios, with the layered UOH-Cd crystallising in a plate morphology and containing a U : Cd : K ratio of 3 : 1.5 : 1. Conversely, the framework-type UOF-Cd incorporates much less Cd, with a U : Cd : K ratio of 4.4 : 0.2 : 1 and is found as needle-like crystals. A common feature in both structures is the presence of β-U3O8 type layers with a distinct uranium centre which lacks the expected uranyl bonds, highlighting the importance of the β-U3O8 layer in the subsequent self-assembly and preferential formation of a variety of structural types. Most importantly, by exploiting the additional flexibility provided by monovalent cation species (i.e., K+) as secondary metal cations to synthesise these novel dual-cation materials, this work highlights the potential for broadening the scope of viable synthetic UOH phases towards furthering the understanding of these systems in their roles as alteration products in the surrounds of spent nuclear fuel in deep geological repositories. [ABSTRACT FROM AUTHOR]

We report a flux crystal growth of Ba2[(UO2)2Ti2O8] and subsequent structural and spectroscopic studies using multiple techniques. The layered crystal structure, built up with sheets of edge‐sharing dimeric uranyl pentagonal bipyramids and dimeric TiO5 square pyramids with interlayer Ba(II) ions, was revealed by synchrotron single crystal x‐ray diffraction and confirmed with electron diffraction using a transmission electron microscope. The presence of only hexavalent uranium was confirmed by both diffuse reflectance and x‐ray absorption near‐edge structure spectroscopies, consistent with the bond valence sum calculations. Its vibrational modes were revealed by Raman spectroscopy. In addition, its implications as a potential hexavalent uranium waste form for the immobilization of uranium‐rich radioactive wastes were discussed. [ABSTRACT FROM AUTHOR]

We report a comprehensive study on lanthanide monouranate–based glass–ceramic (GC) composites as potential waste forms for the immobilization of the lanthanide actinide fraction waste arising from the reprocessing of spent nuclear fuel (SNF). Although the crystalline LnUO4 precursor prepared via a nitrate route can be well stabilized in a sodium aluminoborosilicate glass, the in situ crystallization of EuUO4 in glass from oxide precursors (Eu2O3 and U3O8) is rather robust with regards to various processing conditions such as waste loadings, ceramic‐to‐glass ratios, and cooling rates. Scanning electron microscopy and transmission electron microscopy investigations revealed the detailed microstructures, where ∼1–5‐μm spheres for NdUO4 and EuUO4, and ∼1–5‐μm rectangular crystals for DyUO4 and HoUO4 were observed in residual glasses. As designed, the pentavalent uranium has been confirmed by diffuse reflectance spectroscopy. Overall, LnUO4‐based GC composite waste forms are chemically durable, offering flexible processing options with wide operating windows for SNF and process waste stream management. [ABSTRACT FROM AUTHOR]

Two new mixed-valence uranium oxide hydrate frameworks (UOFs), incorporating either Er3+ or Y3+ ions, were successfully synthesised under hydrothermal conditions and characterised with single-crystal X-ray diffraction and a variety of other structural and spectroscopic techniques. Both frameworks are isostructural and crystallise in the triclinic P1¯ space group, consisting of β-U3O8 type layers pillared by additional uranyl centres, with the Er3+/Y3+ ions lying in the channels of the framework. SEM-EDS analysis found that both materials existed in plate-like morphologies, with a U:Er/Y ratio of 5.5. Bond valence sum analysis revealed the possible existence of pentavalent uranium centres, which was confirmed with diffuse reflectance spectroscopy. Being the first reported UOFs in this space group, this work highlights the complex and flexible nature of these materials, and the broader uranium oxide hydrate systems which exist in the surrounds of spent nuclear fuel disposal in the underground repository. [ABSTRACT FROM AUTHOR]

Uranium oxide hydrate (UOH) materials, a group of minerals and synthetic phases, have attracted recent attention due to their high structural flexibility and diversity as well as their primary relationship with natural weathering of the mineral uraninite and the alteration of spent nuclear fuel (SNF) in geological disposal. Due to the limited structural and chemical understanding of UOH minerals, synthetic UOH phases provide a unique opportunity to fill existing knowledge gaps through the exploration of further structural diversity and distinctive properties, as well as potential applications. Some of the latest developments of synthetic UOH phases include the incorporation of 3d transition metal and lanthanide ions, the evolution of uranyl oxide hydroxide layers driven by interlayer charge, the structural diversity of uranyl oxide hydrate frameworks, and the intrinsic driving force for the formation of diversified structural types. The purpose of this review is to provide a comprehensive summary of the latest advancements of synthetic UOH phases with 3d transition and lanthanide metal ions, including their syntheses, structural diversities, microstructures, uranium valences, vibration modes, and structural and chemical complexities. It also highlights the subsequent implications of these advancements on uranium geochemistry and SNF alterations, amongst other potential applications. A further discussion on technical challenges and knowledge gaps is included to identify areas for future research. [ABSTRACT FROM AUTHOR]

A mixed-valence uranium oxide hydrate framework with Sr2+ ions (UOF-Sr2) was synthesized hydrothermally and characterized with multiple structural and spectroscopic techniques. Compound UOF-Sr2 crystallizes in the monoclinic space group C2/c, having a complicated three-dimensional structure with Sr2+ ions in the narrow channels. Both SEM and TEM characterization revealed a thin-plate crystal morphology, with the TEM-SAED pattern indexed to the crystal structure determined by synchrotron single-crystal XRD. Bond valence sum analysis indicated the possible presence of pentavalent uranium in octahedral coordination environments, subsequently confirmed with diffuse reflectance spectroscopy. Raman spectroscopy revealed extensive splitting of uranyl vibrations due to the presence of various coordination environments for uranyl moieties. This work highlights the complexity and flexibility of the uranium oxide hydrate system in the natural weathering of uraninite, and sheds light on the possible alteration pathways for spent nuclear fuels under geological disposal conditions. [ABSTRACT FROM AUTHOR]

Two uranyl oxide hydrate frameworks (UOFs) incorporating either Eu(III) or Gd(III) ions were synthesized hydrothermally and structurally studied. The uranyl oxide hydroxide layers similar to those in β-U3O8 with both tetragonal and pentagonal bipyramidal uranium polyhedra are connected with pairs of pentagonal bipyramidal uranium polyhedra through uranyl cation–cation interactions to form three-dimensional frameworks with Eu(III) or Gd(III) ions inside the channels. Both SEM and TEM examinations revealed needle crystal morphologies and a U:Eu/Gd ratio of 5.5, with the TEM-SAED pattern indexed to the orthorhombic crystal structure C2221, as also determined using synchrotron single-crystal X-ray diffraction. Raman spectroscopy revealed the band splitting of uranyl symmetric stretching vibrations, reflecting the presence of a unique pentavalent uranium centre in octahedral coordination geometry. The presence of pentavalent uranium in both UOFs was confirmed with diffuse reflectance spectroscopy. Given that layer-structured uranyl oxide hydroxy hydrate phases are dominant for both light and heavy lanthanide ions under similar reaction conditions, the ionic radius plays an important role in controlling the structure types, with UOFs formed only for Eu(III) and Gd(III) ions in the lanthanide series. These new UOFs with lanthanide ions may have various implications especially in nuclear materials. [ABSTRACT FROM AUTHOR]

We report the hydrothermal synthesis and characterization of two uranyl-oxide hydroxy-hydrate compounds with Pr(III) (U–Pr) and Tb(III) (U–Tb) ions prepared via direct hydrothermal reactions of lanthanide (Ln = Pr or Tb) ions with a uranyl-oxide hydroxy-hydrate phase, schoepite. Both compounds U–Pr and U–Tb show thin plate morphologies with atomic ratios of 2 (U : Pr) and 6 (U : Tb) and have been characterized by multiple techniques. The layered structures with interlayer hydrated Pr(III) or Tb(III) ions formed via uranyl–Pr/Tb interactions have been confirmed by synchrotron single crystal X-ray diffraction studies. In addition, the evolution of the uranyl oxide hydroxide layers and anion topologies upon increasing the concentration of interlayer cations by using different U : Ln (Ln = Pr or Tb) ratios has been discussed. The success in the preparation and characterization of compounds U–Pr and U–Tb with different U : Ln (Ln = Pr or Tb) ratios highlights the flexibility of the uranyl oxide hydroxide layers with respect to the incorporation of interlayer cations via a gradual hydroxyl to oxo transition. The study has direct implications in regard to the natural weathering of uraninite mineral and the alteration of spent nuclear fuels during the long-term geological disposal. [ABSTRACT FROM AUTHOR]

Cucurbit[10]uril {Q[10]} has the largest portal size and cavity in the series of Q[n] (n = 5–10) molecules. In contrast to its rich host–guest chemistry, its coordination chemistry is underdeveloped with only limited metal ions being investigated so far. In this work, we initiated the study of Q[10] complexes with Th(IV) and U(VI) ions in HCl solutions via a self-assembly approach. The coordination of Th(IV) ions with Q[10] led to the formation of a compound, {Th4(Cl)16(H2O)20(Q[10])}·nH2O (Q[10]-Th), with a unique nano-tubular structure, while U(VI) ions facilitated the formation of a compound, [(UO2)2(Cl)4(H2O)6]·(Q[10])2·HCl·nH2O (Q[10]-U), with a Q[10]-based supramolecular framework structure via intermolecular outer-surface and second-shell interactions. The structural and spectroscopic aspects of the two compounds together with their optical and thermal properties have been investigated. The successful preparation and characterization of the first two Q[10] compounds with Th(IV)/U(VI) ions highlighted the potential for further exploration of Q[10] coordination chemistry with actinide ions. [ABSTRACT FROM AUTHOR]

Two uranyl peroxide nanoclusters with etidronate ligand (L), a U 20 L 10 and a U 26 L 6 , have been synthesized via self-assembly in aqueous solutions and structurally investigated. Two uranyl peroxide nanoclusters with etidronate ligand [L = (PO 3) 2 C(OH)CH 3 ]4−] have been prepared via self-assembly in aqueous solutions and structurally characterized. K 40 [(UO 2) 20 (O 2) 20 L 10 ]· n H 2 O (1) has a barrel-shaped structure constructed by linking two pentagonal and one decagonal uranyl peroxide rings with ten etidronate ligands. K 38 [(UO 2) 26 (O 2) 33 L 6 ]· n H 2 O (2) has a round top and flat bottom structure constructed by linking a double connected pentagonal uranyl peroxide unit (U 8) and a dome-shaped uranyl peroxide unit (U 18) with six etidronate ligands. The presence of uranyl ion, peroxide and etidronate ligands has been confirmed by Raman spectroscopy and the role of K+ counter ions on the formation of such clusters has been examined. [ABSTRACT FROM AUTHOR]

The synthesis of two zinc-bearing uranium oxide hydrate (UOH) materials has been achieved, and their crystal structures, obtained via single-crystal X-ray diffraction using synchrotron radiation, and additional structural and spectroscopic properties are reported herein. Although both structures incorporate Zn 2+ cations, the two differ significantly. The compound Zn 2 (OH) 2 (H 2 O) 5 [(UO 2 ) 10 UO 14 (H 2 O) 3 ] ( UOHF-Zn ), forming a framework-type structure in the P 1̅ space group, was composed of β-U 3 O 8 layers pillared by uranyl polyhedra, with the Zn 2+ cations incorporated within the framework channels. In contrast, the compound Cs 2 Zn(H 2 O) 4 [(UO 2 ) 4 O 3 (OH) 4 ] 2 ·3H 2 O ( UOH-Zn ) crystallized in the Cmc 2 1 space group with a schoepite-like uranyl oxide hydroxide layered topology and both Zn 2+ and Cs + cations making up the interlayer species. The apparent driving force for the differences in the structures was the change from KOH to CsOH during synthesis, with the smaller K + ions excluded in lieu of a higher proportion of Zn 2+ (U/Zn ratio of 5.5:1) in UOHF-Zn , whereas in UOH-Zn , the larger Cs + ions were preferentially incorporated at the expense of fewer Zn 2+ cations (U/Cs/Zn ratio of 8:2:1). Highlighted in this work is the effect of the chemical species and, in particular, their ionic radius on UOH formation, further improving the understanding of UO 2 alteration in the setting of deep geological repositories., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

We report the hydrothermal syntheses of three uranyl oxide hydroxy-hydrate (UOH) materials containing Sm(III) ions ( UOH-Sm ) by controlling the solution pH and a new way to make Sm-doped porous uranium oxides with different U-to-Sm atomic ratios via their thermal decompositions. While layer-structured UOH-Sm phases with U-to-Sm atomic ratios of 1 ( UOH-Sm1 ) and 4 ( UOH-Sm2 ) were obtained from the reaction of schoepite and samarium nitrate with final solution pH values of over 4, similar reactions without pH adjustment with final solution pH values of less than 4 led to the formation of a uranyl oxide framework ( UOF-Sm ) with a U-to-Sm atomic ratio of 5.5. The crystal structure of compound UOF-Sm was revealed with synchrotron single-crystal X-ray diffraction and confirmed with transmission electron microscopy. The two-dimensional uranyl oxide hydroxide layers, similar to that for β-U 3 O 8 , are linked by double pentagonal uranyl polyhedra to form a three-dimensional framework with Sm(III) ions in the channels. Scanning electron microscopy characterization revealed nanoplate crystal morphologies for the two UOH-Sm phases, in contrast to the needle morphology for UOF-Sm . Subsequent thermal treatments led to the formation of Sm-doped uranium oxides, maintaining the original crystal shapes and U-to-Sm ratios but with nanopores. This work demonstrated that the hydrothermal synthesis conditions, especially fine-tuning of the solution pH, have a significant impact on the uranium hydrolysis, thus leading to well-defined products. This will facilitate the targeted syntheses of UOH phases with lanthanide (Ln) ions and explore the subsequent applications of these materials and Ln-doped porous uranium oxides as potential nuclear or functional materials.

A uranium oxide hydrate framework, [U(H 2 O) 2 ]{[(UO 2 ) 10 O 10 (OH) 2 ][(UO 4 )(H 2 O) 2 ]} ( UOF1 ), was synthesized hydrothermally using schoepite as a uranium precursor. The crystal strucutre of UOF1 was revealed with synchrotron single-crystal X-ray diffraction and confirmed with transmission electron miscroscopy. The typical uranyl oxide hydroxide layers similar to those in β-U 3 O 8 are further connected via double-pentagonal-bipyramidal uranium polyhedra to form a three-dimensional (3D) framework structure with tetravalent uranium species inside the channels. The presence of mixed-valence uranium was investigated with a combination of X-ray absorption near-edge structure and diffuse reflectance spectroscopy. Apart from the major hexavalent uranium, evidence for tetravalent uranium was also found, consistent with the bond valence sum calculations. The successful preparation of UOF1 as the first pure uranium oxide hydrate framework sheds light on the structural understanding of the alteration of UO 2+ x as either a mineral or spent nuclear fuel.