Mild hydrothermal crystal growth of new uranium(IV) fluorides, Na3.13Mg1.43U6F30 and Na2.50Mn1.75U6F30: Structures, optical and magnetic properties

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.

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.

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.

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.

Photocatalytic decomposition of Rhodamine B on uranium-doped mesoporous titanium dioxide

Mesoporous uranium-doped TiO₂ anatase materials were studied to determine the influence of U-doping on the photocatalytic properties for Rhodamine B (RhB) degradation.