Mild Hydrothermal Crystal Growth, Structure, and Magnetic Properties of Ternary U(IV) Containing Fluorides: LiUF5, KU2F9, K7U6F31, RbUF5, RbU2F9, and RbU3F13

Insight into γ-Irradiation-Induced Structure Evolution of Uranium(IV) Germanates as Crystalline Nuclear Waste Forms

New kinds of crystalline waste forms with improved structural stability are desirable for actinide immobilization. In this work, using a molten salt method, two uranium(IV) germanate compounds, namely, K2UGe3O9 (1) and K2UGe2O7 (2), were synthesized, whose compositions consisted of trimeric and dimeric units of germanate, as well as tetravalent uranium, as proved by bond valence calculation and X-ray absorption spectra. Radiation stability assessment is further performed by γ-irradiation to assess the potential of as-synthesized uranium germanate compounds as nuclear waste forms. Powder X-ray diffraction and single-crystal diffraction analyses reveal that 1 remains stable within 1 MGy dosage and undergoes a significant structural change with increasing dosage at 2 MGy, leading to a transformation of 1 to 1-ir analogous to 2 in chemical structure. The underlying mechanism was further studied through a combination of different characterization techniques, including Raman, UV-vis, and electron paramagnetic resonance spectroscopies. Density functional theory calculations of 1 and 2 were also conducted to probe the coordination interaction of germanium and uranium with oxygen atoms. This work reports new crystalline uranium germanates by flux growth and, most importantly, provides insights into the irradiation stability of these materials, which will be beneficial to developing waste forms for long-term immobilization of radionuclides.

Synthesis, Characterization, and Magnetic Properties of Uranium-Containing 2H-Perovskite-Related Sulfides: Ba3MUS6 (M = Mn, Fe, Co, Ni) and Ba3Co0.858(5)Mg0.142(5)US6

The uranium-containing 2H-perovskite-related chalcogenide family of compounds was revisited using the recently developed boron-chalcogen mixture (BCM) method for actinides to aid in their syntheses and to obtain magnetic measurements. Two known 2H-perovskite-related structures, Ba3MnUS6 and Ba3FeUS6, were synthesized using the BCM method and were found to exhibit antiferromagnetic transitions at TN = ∼7.6 and 10.8 K, respectively. Combining the BCM method with the molten flux crystal growth technique resulted in single crystals of three new compositions, Ba3NiUS6, Ba3CoUS6, and Ba3Co0.858(5)Mg0.142(5)US6, the synthesis and characterization of which is reported. Magnetic measurements of Ba3NiUS6 revealed a complex magnetic susceptibility consisting of a weak, glassy, antiferromagnetic transition near 65 K followed by an antiferromagnetic transition at TN = ∼18 K. A reduced radius ratio plot for the existing chalcogenide compositions and new additions to this structure type reported herein is presented to aid in the search for additional 2H-perovskite-related sulfides.

Comparative Study on Crystal Structures and Synthetic Techniques of Ternary Hafnium/Zirconium Fluorides

Mild hydrothermal synthesis was employed to grow high-quality single crystals of ternary fluoridohafnates at low temperatures. The series of new materials was characterized using single-crystal X-ray diffraction, and the crystal structures of AHfF₆ (A = Mg and Sr), A₂HfF₈ (A = Ba and Pb), Ca₅Hf₃F₂₂, and Cd₂HfF₈(H₂O)₆ are discussed herein. Although some material compositions have similar stoichiometries, all of the compositions adopt different structural motifs. A comparison of the crystal structures and synthesis techniques of ternary fluoridohafnates and ternary fluoridozirconates is also reported, and the impact of the subtle changes of synthesis conditions on overall structures is discussed.

Chloride Reduction of Mn3+ in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMn2+Mn3+F6, a Canted Antiferromagnet with a Hard Ferromagnetic Component

Geometrically frustrated systems play an important role in studying new physical phenomena and unconventional thermodynamics. Charge ordered defect pyrochlores AM²⁺M³⁺F₆ offer a convenient platform for probing the interplay between electron distribution over M²⁺ and M³⁺ sites and structural distortions; however, they are limited to compounds with M^2+/3+ = V, Fe, Ni, and Cu due to difficulties in the simultaneous stabilization of other 3d elements in the +2 and +3 oxidation states. Herein, we employ Cl^- anions under hydrothermal conditions for the mild reduction of Mn₂O₃ in concentrated HF to obtain the CsMn²⁺Mn³⁺F₆ composition as a phase pure sample and study its properties. The magnetism of CsMn₂F₆ was characterized by measuring the magnetic susceptibility and isothermal magnetization data, and a magnetic transition to a canted antiferromagnet state was found at 24.1 K. We determined the magnetic structure of CsMn₂F₆ using powder neutron diffraction, which revealed successive long-range ordering of the Mn²⁺ and Mn³⁺ sites that is accompanied by a second transition. The role and strength of magnetic exchange interactions were characterized using DFT calculations.

Examination of Molten Salt Reactor Relevant Elements Using Hydrothermal Synthesis

The structural chemistry of elements relevant to the FLiBe molten salt reactor, Th, U, Np, and Zr, including Ce and Nd (as analogues for Pu and Am, respectively), have been examined using hydrothermal synthesis at 200 °C. These reactions serve to model the reaction of molten salts under hydrolysis conditions. The results show that U and Np formed LiAnF₅, while Ce formed Li₄CeF₈. The source of U also controlled the crystal quality, where UO₂ gave small crystals, while UO₃·2H₂O gave very large crystals. It is likely that Be incorporation was not observed because of the high solubility of [BeF₄]^2- in water. Zr formed a third product, Li₆BeF₄ZrF₈, which features isolated [BeF₄]^2- and [ZrF₈]^4- units bridged by Li⁺. Additionally, Li₂BeF₄ was regularly isolated. When little to no alkali metal was included in the reaction, M₃F₁₂(H₂O) was isolated for Np, U, and Ce.

Structure, Spectroscopy, and Theoretical Analysis of Zero- and Three-Dimensional Lithium Plutonium Fluorides: Li4PuF8 and LiPuF5

The reaction of ²⁴²PuO₂ with HF and LiF under hydrothermal conditions results in the formation of Li₄PuF₈ and LiPuF₅. These compounds were structurally characterized using single-crystal X-ray diffraction, and UV-vis-near-IR absorption spectroscopy was employed to confirm the oxidation state of the plutonium in the compounds as 4+. The structure of Li₄PuF₈ consists of [PuF₈]^4- anions that adopt a bicapped trigonal-prismatic geometry with approximate C_2v symmetry. These molecules are bridged by Li⁺ cations. In contrast, LiPuF₅ forms a dense three-dimensional network constructed from [PuF₉]^5- units that are bridged by F^- anions. The Pu⁴⁺ cations are found within tricapped trigonal prisms. Extensive theoretical analysis of the electronic and bonding interactions is included with a comparison between the results derived from complete-active-space self-consistent-field at different levels of theory, quantum theory of atoms in molecules, interacting quantum atom, natural localized molecular orbital, and Wiberg bond order analyses. Covalent interactions in these compounds are examined, and intramolecular trends in covalent and electrostatic interactions are discussed.

The low-temperature barium fluoridozirconate variety α-BaZr2F10

The low-temperature triclinic variety α-BaZr₂F₁₀ constitutes a new structure type, less symmetrical than the higher-temperature β-variety. It is based on the stacking of double sheets of Zr polyhedra, connecting three different kinds of ZrF₇ polyhedra and one ZrF₈ polyhedron via vertices and edges, separated by corrugated Ba²⁺ layers. It is compared to the high-temperature β-variety, directly recrystallizing from barium fluoridozirconate glass, and also to BaTe₂F₁₀ and KTe₂F₉.

Breaking a Paradigm: Observation of Magnetic Order in the Purple U(IV) Phosphite: U(HPO3)2

The hydrothermal crystal growth of a new uranium phosphite, U(HPO₃)₂, is reported. This material was found to exhibit optical and magnetic properties not commonly observed in U(IV) containing materials, specifically purple coloration and low temperature magnetism. We discuss the synthesis, structure determination, and characterization of the title compound and comment on its optical, thermal, and magnetic properties. The magnetic behavior is consistent with frustrated antiferromagnetism that arises from the hexagonal honeycomb lattice of U(IV) ions. This material gives rare evidence for U(IV) ions participating in magnetic order.

Retention of a Paramagnetic Ground State at Low Temperatures in a Family of Structurally Related UIV Phosphates

A new uranium fluoride phosphate, UFPO₄, was synthesized via a mild hydrothermal route and characterized optically, thermally, and magnetically. Two thermal transformation products, U₂O(PO₄)₂ and U^IVU^VIO₂(PO₄)₂, were discovered to be structurally related, and were subsequently synthesized for bulk property measurements. All three materials failed to follow Curie-Weiss behavior at low temperatures, attributed to the nearly ubiquitous singlet ground state of U(IV), transitioning into a Curie-Weiss paramagnetic regime at high temperatures. Neutron diffraction experiments were performed on UFPO₄ and U^IVU^VIO₂(PO₄)₂ in order to characterize this unusual magnetic behavior.

Expanding pentafluorouranates: hydrothermal synthesis and characterization of β-NaUF5 and β-NaUF5·H2O

Two new sodium uranium(iv) pentafluorides eliminate the necessity of a copper catalyst in the synthesis of alkali metal uranium fluorides.

Application of a mild hydrothermal method to the synthesis of mixed transition-metal(ii)/uranium(iv) fluorides

The synthesis and properties of two families of mixed metal U(iv)/M(ii) fluorides via a mild hydrothermal method.

Synthetic strategies for new vanadium oxyfluorides containing novel building blocks: structures of V(iv) and V(v) containing Sr4V3O5F13, Pb7V4O8F18, Pb2VO2F5, and Pb2VOF6

Four novel vanadium oxyfluorides (VOFs) featuring new crystallographic building blocks were crystallizedviaa mild hydrothermal route.