Expansion of the Rich Structures and Magnetic Properties of Neptunium Selenites: Soft Ferromagnetism in Np(SeO3)2

Flux Synthesis of Alkaline-Earth Lanthanide Borates as Potential Nuclear Waste Forms

Neodymium is typically considered the best surrogate for trivalent americium and can be used to identify Am3+ containing materials that are likely to form. We have explored the alkaline-earth lanthanide borate phase space using alkaline-earth halide/carbonate fluxes. This resulted in the synthesis of new compounds AE5Ln(BO3)4X (AE = Ca, Sr; Ln = Pr, Nd, Eu, Tb; X = Cl, Br) and AE3Ln2(BO3)4 (AE = Sr, Ba; Ln = Pr, Nd) as well as the synthesis of two compounds of Ba8Ln2(BO3)6(B2O5) (Ln = Eu, Tb) crystallizing in a new structure type. Ba8Ln2(BO3)6(B2O5) crystallizes in the space group P21/n with lattice parameters a = 8.6002(3) Å, b = 7.9245(3) Å, c = 17.6697(7) Å, and β = 91.3560(10)° for the Eu analogue, and the structure contains isolated LnO8 polyhedra connected into a framework by BO3 and B2O5 units. The fluorescence emission spectra of AE5Ln(BO3)4X (AE = Ca, Sr; Ln = Eu, Tb; X = Cl, Br) and Ba8Ln2(BO3)6(B2O5) (Ln = Eu, Tb) are reported.

Crystal Chemistry and Structural Complexity of Natural and Synthetic Uranyl Selenites

Comparison of the natural and synthetic phases allows an overview to be made and even an understanding of the crystal growth processes and mechanisms of the particular crystal structure formation. Thus, in this work, we review the crystal chemistry of the family of uranyl selenite compounds, paying special attention to the pathways of synthesis and topological analysis of the known crystal structures. Comparison of the isotypic natural and synthetic uranyl-bearing compounds suggests that uranyl selenite mineral formation requires heating, which most likely can be attributed to the radioactive decay. Structural complexity studies revealed that the majority of synthetic compounds have the topological symmetry of uranyl selenite building blocks equal to the structural symmetry, which means that the highest symmetry of uranyl complexes is preserved regardless of the interstitial filling of the structures. Whereas the real symmetry of U-Se complexes in the structures of minerals is lower than their topological symmetry, which means that interstitial cations and H2O molecules significantly affect the structural architecture of natural compounds. At the same time, structural complexity parameters for the whole structure are usually higher for the minerals than those for the synthetic compounds of a similar or close organization, which probably indicates the preferred existence of such natural-born architectures. In addition, the reexamination of the crystal structures of two uranyl selenite minerals guilleminite and demesmaekerite is reported. As a result of the single crystal X-ray diffraction analysis of demesmaekerite, Pb2Cu5[(UO2)2(SeO3)6(OH)6](H2O)2, the H atoms positions belonging to the interstitial H2O molecules were assigned. The refinement of the guilleminite crystal structure allowed the determination of an additional site arranged within the void of the interlayer space and occupied by an H2O molecule, which suggests the formula of guilleminite to be written as Ba[(UO2)3(SeO3)2O2](H2O)4 instead of Ba[(UO2)3(SeO3)2O2](H2O)3.

A Large Family of Centrosymmetric and Chiral f-Element-Bearing Iodate Selenates Exhibiting Coordination Number and Dimensional Reductions

The exploration of phase formation in the f-element-bearing iodate selenate system has resulted in 14 novel rare-earth-containing iodate selenates, Ln(IO₃)(SeO₄) (Ln = La, Ce, Pr, Nd; LnISeO-1), Ln(IO₃)(SeO₄)(H₂O) (Ln = Sm, Eu; LnISeO-2), and Ln(IO₃)(SeO₄)(H₂O)₂·H₂O (Ln = Gd, Dy, Ho, Er, Tm, Yb, Lu, Y; LnISeO-3), as well as two new thorium iodate selenates, Th(OH)(IO₃)(SeO₄)(H₂O) (ThISeO-1) and Th(IO₃)₂(SeO₄) (ThISeO-2). LnISeO-3 and ThISeO-2 crystallize in the chiral space group P2₁2₁2₁, while LnISeO-1, LnISeO-2, and ThISeO-1 crystallize in the centrosymmetric space group P2₁/c. The numbers of both coordinating and hydrating water molecules crystallized in LnISeO-1, LnISeO-2, and LnISeO-3 increase along these three series, in line with the increasingly negative values of hydration enthalpies of heavier trivalent lanthanide ions. Such a systematic change in compositions, especially the first coordination sphere of Ln, further induces structural rearrangements, including coordination number and dimensional reductions. More specifically, the structures of LnISeO-1, LnISeO-2, and LnISeO-3 have undergone transitions from 2D Ln-oxo layers with 10-coordinate Ln centers to 1D Ln-oxo chains with 9-coordinate Ln centers and then to 0D Ln-oxo monomers with 8-coordinate Ln centers, respectively. The formation and characterization of this large family of Ln/Th iodate selenates suggest that such a mixed-anion system not only exhibits richer structural chemistry but also can be capable of generating intriguing properties, such as the second-harmonic generation (SHG) effect.

Unexpected Behavior of Np in Oxo-selenate/Oxo-selenite Systems

A study of neptunium (Np) chemistry in the complex oxo-selenium system has been performed. Hereby, two sets of precipitation experiments were conducted, investigating the influence of the initial oxidation state of selenium using Se^IVO₂ and H₂Se^VIO₄ with Np^V in alkali nitrate solution, keeping the ratio of Np/Se constant. Surprising results were observed. Five novel neptunium and selenium bearing compounds have been obtained by slow evaporation from aqueous solution. The novel Np^IV phase K_4-x[Np(SeO₃)_4-x(HSeO₃)_x]·(H₂O)_1.5 (1) crystallizes in green-colored, plate-shaped crystals and was obtained by adding SeO₂ and ANO₃ to a Np^V stock solution. Single-crystal X-ray diffraction reveals one-dimensional chain structures composed of square antiprismatic NpO₈ polyhedra linked via four trigonal pyramidal SeO₃ and HSeO₃ units. Raman spectral analysis supports the presence of both selenite and hydroselenite due to the presence of corresponding modes within the spectra. The addition of selenic acid to a Np^V stock solution resulted in the precipitation of elongated rose prisms of K₂[(NpO₂)₂(SeO₄)₃(H₂O)₂]·(H₂O)_1.5 (2), Rb₂[(NpO₂)₂(SeO₄)₃(H₂O)₂]·(H₂O)₂ (3) and K₉[(NpO₂)₉(SeO₄)_13.5(H₂O)₆]·(H₂O)₁₂ (4) as well as light red plates of Cs₂[(NpO₂)₂(SeO₄)₃] (5). To our knowledge, this is the first report of Np^VI selenates. All four structures show two-dimensional layered structures with alkali cations acting as charge balancing counter cations. Hereby the layers of compounds 2 and 3 are found to be orientational geometric isomers. Distinctly different phenomena are made responsible for the phase formation within these systems. The kinetically driven process of Np^V disproportionation led to the formation of the Np^IV selenites in the Se^IV-based system, whereas the oxidation of Np^V by reduction of nitrate in acidic conditions is responsible for the formation of the Np^VI selenates in the Se^VI system. The influence of air oxygen is also discussed for the latter reaction.