Application of a Mild Hydrothermal Approach Containing an in Situ Reduction Step to the Growth of Single Crystals of the Quaternary U(IV)-Containing Fluorides Na4MU6F30 (M = Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) Crystal Growth, Structures, and Magnetic Properties

Exploring the Valence Diversity of Uranium by Flux Growth of Uranium Silicate under Inert Atmosphere

The attributes of good solubility and the redox-neutral nature of molten salt fluxes enable them to be useful for the synthesis of novel crystalline actinide compounds. In this work, a flux growth method under an inert atmosphere is proposed to explore the valence diversity of uranium, and a series of five uranium silicate structures, [K3Cl][(UVIO2)(Si4O10)] (1), Cs3[(UVO2)(Si4O10)] (2), K2[UIV(Si2O7)] (3), K8[(UVIO2)(UVO2)2(Si8O22)] (4), and Cs6[UIV(UVO)2(Si12O32)] (5), were synthesized using different metal halide salt and feeding U/Si ratios. Crystal structure analysis reveals that the utilization of argon atmosphere that helps to avoid possible oxidation of low-valence uranium generates a variety of oxidation states of uranium including U(VI), U(V), U(IV), mixed-valence U(V) and U(VI), and mixed-valence U(IV) and U(V). Characterization of physicochemical properties of representative compounds shows that all these uranium silicate compounds have bandgaps among the range of 2.0-3.4 eV, and mixed-valence uranium silicate compounds have relatively narrower bandgaps. Density functional theory calculations on formation enthalpies, lattice energies, and bandgaps of all five compounds were also performed to provide more structural information about these uranium silicates. This work enriches the library of variable-valence uranium silicate compounds and provides a feasible way to produce novel actinide compounds with intriguing properties through the flux growth method that might show potential application in relevant fields such as storage media for nuclear waste.

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.

Developing Waste Forms for Transuranic Elements: Quaternary Neptunium Fluorides of the Type NaxMNp6F30 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Al, Ga)

A series of quaternary Np(IV) fluorides was synthesized using a mild hydrothermal synthesis approach. The compositions are all of the type Na_xMNp₆F₃₀, where M = Ti(III), V(III), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Al(III), and Ga(III) and x = 4 for divalent metals, x = 3 for trivalent metals. The compounds all crystallize in the P-3c1 space group and are isotypic with actinide analogues Na_xMAn₆F₃₀ (An = Ce, U, Th, Pu). Structure data from the neptunium crystals were combined with data from the other actinide analogues to establish the tetravalent, nine-coordinated ionic radii of neptunium (1.030(2) Å), plutonium (1.014(1) Å), and cerium (1.012(2) Å). Radiation damage studies were also carried out on a surrogate material, the cerium analogue Na₃AlCe₆F₃₀, which indicates that the structure type has low resistance to amorphization. Density functional theory calculations were carried out to compute the band gaps and enthalpies of formation variations among the isotypic cerium and actinide structures to compare the stability of the structures.

Structures and Magnetic Properties of K2Pd4U6S17, K2Pt4U6S17, Rb2Pt4U6S17, and Cs2Pt4U6S17 Synthesized Using the Boron-Chalcogen Mixture Method

A series of A₂M₄U₆S₁₇ (A = alkali metal; M = Pd, Pt) compounds, specifically K₂Pd₄U₆S₁₇, K₂Pt₄U₆S₁₇, Rb₂Pt₄U₆S₁₇, and Cs₂Pt₄U₆S₁₇, were synthesized using the combined boron-chalcogen mixture and molten flux crystal growth methods. The formation of rubidium- and cesium-containing analogues resulted from a in situ alkali polysulfide flux formed from alkali carbonates. The successful synthesis of single crystals of the title compounds allowed for their structural characterization by single-crystal X-ray diffraction. The structure determination revealed disorder of the alkali cations in Rb₂Pt₄U₆S₁₇ and Cs₂Pt₄U₆S₁₇, while the potassium cations in K₂Pd₄U₆S₁₇ and K₂Pt₄U₆S₁₇ were fully ordered. Magnetic measurements were performed on samples of K₂Pt₄U₆S₁₇, Rb₂Pt₄U₆S₁₇, and Cs₂Pt₄U₆S₁₇ that contained small amounts of paramagnetic β-US₂ and diamagnetic PtS. Antiferromagnetic order was observed at T_N = 9.1 K for K₂Pt₄U₆S₁₇. No long-range magnetic order was observed for Rb₂Pt₄U₆S₁₇ and Cs₂Pt₄U₆S₁₇. Uranium moments of 2.5, 2.6, and 2.6 μ_B were measured for K₂Pt₄U₆S₁₇, Rb₂Pt₄U₆S₁₇, and Cs₂Pt₄U₆S₁₇, respectively.

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.

Expansion of the Na3 MIII (Ln/An)6 F30 Series: Incorporation of Plutonium into a Highly Robust and Stable Framework

Na_n MAn₆ F₃₀ is an extremely versatile framework structure for incorporating tetravalent actinides (An) and cerium along with divalent or trivalent d-metals (M); moreover, the structure exhibits a high resistance to harsh chemical conditions. This extreme robustness can potentially be exploited for the sequestration of plutonium in a stable matrix; however, no Na_n MPu₆ F₃₀ compounds have been reported so far. Herein, we present four new plutonium fluorides that have been prepared as single crystals by mild hydrothermal synthesis methods. Structural characterizations revealed their compositions to be Na₃ AlPu₆ F₃₀ , Na₃ FePu₆ F₃₀ , Na₃ CoPu₆ F₃₀ , and Na_2.4 Mn_1.6 Pu₆ F₃₀ . Surprisingly, in the plutonium series, it was found that Co²⁺ and Mn²⁺ precursors oxidized to form Na₃ Co^III Pu₆ F₃₀ and Na_2.4 Mn^II/III _1.6 Pu₆ F₃₀ , whereas the analogous reactions for cerium result in reduction of the transition metal, even when beginning with a M³⁺ precursor. While cerium is often used as a surrogate for plutonium, this work serves as an example that deviations between their chemistries do occur.

[U(H2O)2]{[(UO2)10O10(OH)2][(UO4)(H2O)2]}: A Mixed-Valence Uranium Oxide Hydrate Framework

A uranium oxide hydrate framework, [U(H₂O)₂]{[(UO₂)₁₀O₁₀(OH)₂][(UO₄)(H₂O)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-U₃O₈ 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.

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.

Quaternary cerium(iv) containing fluorides exhibiting Ce3F16 sheets and Ce6F30 frameworks

A view of the size of trivalent cations M³⁺ plotted against the size of the tetravalent framework forming cations Th⁴⁺, U⁴⁺, and Ce⁴⁺ and a complex 3D framework structure of Na₃MCe₆F₃₀ (M = Al, Ga, Fe, and Cr) along the c-axes.

Uranyl oxide hydrate frameworks with lanthanide ions

The first two uranyl oxide hydrate frameworks incorporating lanthanide ions (Ln = Eu³⁺/Gd³⁺) have been synthesized hydrothermally and characterized.

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.

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.

An Ultrastable Heterobimetallic Uranium(IV)/Vanadium(III) Solid Compound Protected by a Redox-Active Phosphite Ligand: Crystal Structure, Oxidative Dissolution, and First-Principles Simulation

The first heterobimetallic uranium(IV)/vanadium(III) phosphite compound, Na₂UV₂(HPO₃)₆ (denoted as UVP), was synthesized via an in situ redox-active hydrothermal reaction. It exhibits superior hydrolytic and antioxidant stability compared to the majority of structures containing low-valent uranium or vanadium, further elucidated by first-principles simulations, and therefore shows potential applications in nuclear waste management.

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.

Influence of extreme conditions on the formation and structures of caesium uranium(VI) arsenates

Four new uranyl arsenates, Cs2[(UO2)(As2O7)] (1), α-Cs[(UO2)(HAs2O7)] (2), β-Cs[(UO2)(HAs2O7)] (3), Cs[(UO2)(HAs2O7)]·0.17H2O (4), were synthesized by high-temperature/high pressure (HT/HP) reactions at 900 °C and 3 GPa. These phases were subsequently characterised structurally as well as chemically. We demonstrated that compound 1 can also be obtained at ambient pressure. Compounds 1, 2, and 4 are based on two-dimensional (2D) anionic layers with two different topological types. The layers possess a similar composition, [(UO2)(As2O7)](2-) in 1 and [(UO2)(HAs2O7)](-) in 2 and 4. However, the presence of hydrogen in 2 and 4 leads to a change in coordination modes of the pyroarsenate groups. There are additional 0.17 H2O molecules per formula unit in 4, which cause slight distortions of the layers in 4. All these layers can be simplified to a common net, which is typical of autunite-like layered compounds. Compound 3 is a polymorph of compound 2, but the structural arrangements in these two are significantly different. The structure of 3 is based upon a three-dimensional (3D) framework, in which UO7 is coordinated by arsenate groups in order to form uranyl anion sheets, and UO6 is located within the interlayers. Bond valance analysis proved the presence of OH(-) groups in compounds 2, 3, and 4, respectively, and water molecules in 4. The Raman analyses enabled the study of the local environments of the arsenate and the uranyl groups within the investigated phases, respectively. It turned out that the applied HT/HP synthesis method strongly affects the crystal chemistry as well as the observed structural features of all obtained compounds.

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.