Flux Synthesis, Structure, Properties, and Theoretical Magnetic Study of Uranium(IV)-Containing A2USi6O15 (A = K, Rb) with an Intriguing Green-to-Purple, Crystal-to-Crystal Structural Transition in the K Analogue

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.

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.

Supramolecular Assembly of U(IV) Clusters and Superatoms with Unconventional Countercations

Superatoms are nanometer-sized molecules or particles that form ordered lattices, mimicking their atomic counterparts. Hierarchical assembly of superatoms gives rise to emergent properties in lattices of quantum dots, p-block clusters, and fullerenes. Here, we introduce a family of uranium-oxysulfate cluster anions whose hierarchical assembly in water is controlled by two parameters: acidity and the lanthanide or transition-metal countercation. In acid, larger Ln^III (Ln = La-Ho) link hexamer (U₆) oxoclusters into body-centered cubic frameworks, while smaller Ln^III (Ln = Er-Lu and Y) promote linking of 14 U₆ clusters into hollow superclusters (U₈₄ superatoms). U₈₄ assembles into superlattices including cubic-closest packed, body-centered cubic, and interpenetrating networks, bridged by interstitial countercations and U₆ clusters. Divalent transition metals (TM = Mn^II and Zn^II) charge-balance and promote the fusion of 10 U₆ and 10 U monomers into a wheel-shaped cluster (U₇₀). Dissolution of U₇₀ in organic media reveals (by small-angle X-ray scattering) that differing supramolecular assemblies are accessed, controlled by TM^II-linking of U₇₀ clusters. Magnetic measurements of these assemblies reveal Curie-Weiss behavior at high temperatures, without pairing of the 5f²-electrons down to 2 K.

Targeting complex plutonium oxides by combining crystal chemical reasoning with density-functional theory calculations: the quaternary plutonium oxide Cs2PuSi6O15

The stability of the novel Pu(iv) silicate, Cs₂PuSi₆O₁₅, was predicted from a combination of crystal chemical reasoning and DFT calculations and confirmed by its synthesis via flux crystal growth.

Structural and Spectroscopic Investigation of Novel 2D and 3D Uranium Oxo-Silicates/Germanates and Some Statistical Aspects of Uranyl Coordination in Oxo-Salts

Synthesis, structural and spectroscopic characterization, and topological analysis of five novel uranyl-based silicates and germanates have been performed. The open-framework K₄(UO₂)₂Si₈O₂₀·4H₂O has been synthesized under hydrothermal conditions and is based upon [USi₆] heptamers interconnected via edge-sharing. Its structure is composed of sechser silicate layers with 4-, 8-, and 16-membered rings. The largest 16-membered rings have an average dimension of ∼8.93 × 9.42 Ų. β-K₂(UO₂)Si₄O₁₀ has been obtained by the high-temperature flux growth method. Its 3D framework contains a loop-branched sechser single layer with 4- and 8-membered rings and consists of the same [USi₆] heptamers as observed in K₄(UO₂)₂Si₈O₂₀·4H₂O. Na₆(UO₂)₃(Si₂O₇)₂ has also been synthesized from melted fluxes and represents a 2D layer structure composed by [USi₄] pentamers. Two iso-structural compounds A⁺(UO₂)(HGeO₄)·H₂O (A⁺ = Rb⁺, Cs⁺) were synthesized via the hydrothermal method, and their structures are of the α-uranophane type. The 2D layers consist of [U₂Ge₂] tetramer secondary building units (SBUs). The Raman spectra of all novel phases were collected, and bands were assigned according to the existing oxo-silicate rings and oxo-germanium units. Additionally, we performed a statistical investigation of the local coordination of uranyl ions in all known inorganic structures with different oxo-anions (TO_x, T = B³⁺, Si/Ge⁴⁺, P/As⁵⁺, S/Se/Te⁶⁺, Cr/Mo/W⁶⁺, P/As³⁺, and Se/Te⁴⁺). We found a direct correlation between the ionic potential of the central cations T in oxo-anions in their higher oxidation states and the coordination number of uranyl groups.

Flux crystal growth: a versatile technique to reveal the crystal chemistry of complex uranium oxides

This frontier article focuses on the use of flux crystal growth for the preparation of new actinide containing materials, reviews the history of flux crystal growth of uranium containing phases, and highlights the recent advances in the field. Specifically, we discuss how recent developments in f-element materials, fueled by accelerated materials discovery via crystal growth, have led to the synthesis and characterization of new families of complex uranium containing oxides, namely alkali/alkaline uranates, oxychlorides, oxychalcogenides, tellurites, molybdates, tungstates, chromates, phosphates, arsenates, vanadates, niobates, silicates, germanates, and borates. An overview of flux crystal growth is presented and specific crystal growth approaches are described with an emphasis on how and why they - versus some other method - are used and how they enable the preparation of specific classes of new materials.

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.

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.

One-dimensional chain structures of hexanuclear uranium(iv) clusters bridged by formate ligands

Three one-dimensional chain structures of uranium(iv) hexanuclear clusters have been synthesized under hydrothermal/solvothermal conditions.

On the calculation of multiplet energies of three-open-shell 4f135fn6d1electron configuration by LFDFT: modeling the optical spectra of 4f core-electron excitation in actinide compounds

My presentation relates the modeling of X-ray absorption spectra of actinides, exemplified here by the study of U⁴⁺ion with configuration 4f¹³5f²6d¹.

Development and applications of the LFDFT: the non-empirical account of ligand field and the simulation of the f-d transitions by density functional theory

Ligand field density functional theory (LFDFT) is a methodology consisting of non-standard handling of DFT calculations and post-computation analysis, emulating the ligand field parameters in a non-empirical way. Recently, the procedure was extended for two-open-shell systems, with relevance for inter-shell transitions in lanthanides, of utmost importance in understanding the optical and magnetic properties of rare-earth materials. Here, we expand the model to the calculation of intensities of f → d transitions, enabling the simulation of spectral profiles. We focus on Eu(2+)-based systems: this lanthanide ion undergoes many dipole-allowed transitions from the initial 4f(7)((8)S7/2) state to the final 4f(6)5d(1) ones, considering the free ion and doped materials. The relativistic calculations showed a good agreement with experimental data for a gaseous Eu(2+) ion, producing reliable Slater-Condon and spin-orbit coupling parameters. The Eu(2+) ion-doped fluorite-type lattices, CaF2:Eu(2+) and SrCl2:Eu(2+), in sites with octahedral symmetry, are studied in detail. The related Slater-Condon and spin-orbit coupling parameters from the doped materials are compared to those for the free ion, revealing small changes for the 4f shell side and relatively important shifts for those associated with the 5d shell. The ligand field scheme, in Wybourne parameterization, shows a good agreement with the phenomenological interpretation of the experiment. The non-empirical computed parameters are used to calculate the energy and intensity of the 4f(7)-4f(6)5d(1) transitions, rendering a realistic convoluted spectrum.