Theoretical Study of the Structure and Bonding in ThC₂ and UC₂

The electronic structure and various molecular properties of the actinide (An) dicarbides ThC₂ and UC₂ were investigated by relativistic quantum chemical calculations. We probe five possible geometrical arrangements: two triangular structures including an acetylide (C₂) moiety, as well as the linear AnCC, CAnC, and bent CAnC geometries. Our calculations at various levels of theory indicate that the triangular species are energetically more favorable, while the latter three arrangements proved to be higher-energy structures. Our SO-CASPT2 calculations give the ground-state molecular geometry for both ThC₂ and UC₂ as the symmetric (<i>C</i>_2<i>v</i>) triangular structure. The similar and, also very close in energy, asymmetric (<i>C</i>_<i>s</i>) triangular geometry belongs to a different electronic state. DFT and single-determinant ab initio methods failed to distinguish between these two similar electronic states demonstrating the power of multiconfiguration ab initio methods to deal with such subtle and delicate problems. We report detailed data on the electronic structure and bonding properties of the most relevant structures

Structural Properties and Charge Distribution of the Sodium Uranium, Neptunium, and Plutonium Ternary Oxides: A Combined X‑ray Diffraction and XANES Study

The charge distributions in α-Na₂UO₄, Na₃NpO₄, α-Na₂NpO₄, Na₄NpO₅, Na₅NpO₆, Na₂PuO₃, Na₄PuO₅, and Na₅PuO₆ are investigated in this work using X-ray absorption near-edge structure (XANES) spectroscopy at the U-L₃, Np-L₃, and Pu-L₃ edges. In addition, a Rietveld refinement of monoclinic Na₂PuO₃, in space group <i>C</i>2/<i>c</i>, is reported for the first time, and the existence of the isostructural Na₂NpO₃ phase is revealed. In contrast to measurements in solution, the number of published XANES data for neptunium and plutonium solid phases with a valence state higher than IV is very limited. The present results cover a wide range of oxidation states, namely, IV to VII, and can serve as reference for future investigations. The sodium actinide series show a variety of local coordination geometries, and correlations between the shape of the XANES spectra and the local structural environments are discussed herein

Optimization of Uranium-Doped Americium Oxide Synthesis for Space Application

Americium 241 is a potential alternative to plutonium 238 as an energy source for missions into deep space or to the dark side of planetary bodies. In order to use the ²⁴¹Am isotope for radioisotope thermoelectric generator or radioisotope heating unit (RHU) production, americium materials need to be developed. This study focuses on the stabilization of a cubic americium oxide phase using uranium as the dopant. After optimization of the material preparation, (Am_0.80U_0.12Np_0.06Pu_0.02)­O_1.8 has been successfully synthesized to prepare a 2.96 g pellet containing 2.13 g of ²⁴¹Am for fabrication of a small scale RHU prototype. Compared to the use of pure americium oxide, the use of uranium-doped americium oxide leads to a number of improvements from a material properties and safety point of view, such as good behavior under sintering conditions or under alpha self-irradiation. The mixed oxide is a good host for neptunium (i.e., the ²⁴¹Am daughter element), and it has improved safety against radioactive material dispersion in the case of accidental conditions

Triclinic–Cubic Phase Transition and Negative Expansion in the Actinide IV (Th, U, Np, Pu) Diphosphates

The <i>An</i>P₂O₇ diphosphates (<i>An</i> = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the <i>An</i>^IV cation and its suitability for the unusual octahedral environment. Synchrotron and X-ray diffraction, thermal analysis, Raman spectroscopy, and ³¹P nuclear magnetic resonance reveal them as a new family of diphosphates which probably includes the recently studied CeP₂O₇. Although they adopt at high temperature the same cubic archetypal cell as the other known MP₂O₇ diphosphates, they differ by a very faint triclinic distortion at room temperature that results from an ordering of the P₂O₇ units, as shown using high-resolution synchrotron diffraction for UP₂O₇. The uncommon triclinic–cubic phase transition is first order, and its temperature is very sensitive to the ionic radius of <i>An</i>^IV. The conflicting effects which control the thermal variations of the P–O–P angle are responsible for a strong expansion of the cell followed by a contraction at higher temperature. This inversion of expansion occurs at a temperature significantly higher than the phase transition, at variance with the parent compounds with smaller M^IV cations in which the two phenomena coincide. As shown by various approaches, the P–O_b–P linkage remains bent in the cubic form

X‑ray Diffraction, Mössbauer Spectroscopy, Magnetic Susceptibility, and Specific Heat Investigations of Na₄NpO₅ and Na₅NpO₆

The hexavalent and heptavalent sodium neptunate compounds Na₄NpO₅ and Na₅NpO₆ have been investigated using X-ray powder diffraction, Mössbauer spectroscopy, magnetic susceptibility, and specific heat measurements. Na₄NpO₅ has tetragonal symmetry in the space group <i>I</i>4/<i>m</i>, while Na₅NpO₆ adopts a monoclinic unit cell in the space group <i>C</i>2/<i>m</i>. Both structures have been refined for the first time using the Rietveld method. The valence states of neptunium in these two compounds, i.e., Np­(VI) and Np­(VII), respectively, have been confirmed by the isomer shift values of their Mössbauer spectra. The local structural properties obtained from the X-ray refinements have also been related to the quadrupole coupling constants and asymmetry parameters determined from the Mössbauer studies. The absence of magnetic ordering has been confirmed for Na₄NpO₅. However, specific heat measurements at low temperatures have suggested the existence of a Schottky-type anomaly at around 7 K in this Np­(VI) phase

A New Look at the Structural Properties of Trisodium Uranate Na₃UO₄

The crystal structure of trisodium uranate, which forms following the interaction between sodium and hyperstoichiometric urania, has been solved for the first time using powder X-ray and neutron diffraction, X-ray absorption near-edge structure spectroscopy, and solid-state ²³Na multiquantum magic angle spinning nuclear magnetic resonance. The compound, isostructural with Na₃BiO₄, has monoclinic symmetry, in space group <i>P</i>2/<i>c</i>. Moreover, it has been shown that this structure can accommodate some cationic disorder, with up to 16(2)% sodium on the uranium site, corresponding to the composition α-Na₃(U_1–<i>x</i>,Na_<i>x</i>)­O₄ (0 < <i>x</i> < 0.18). The α phase adopts a mixed valence state with the presence of U­(V) and U­(VI). The two polymorphs of this compound described in the literature, <i>m</i>- and β-Na₃(U_1–<i>x</i>,Na_<i>x</i>)­O₄, have also been investigated, and their relationship to the α phase has been established. The completely disordered low-temperature cubic phase corresponds to a metastable phase. The semiordered high-temperature β phase is cubic, in space group <i>Fd</i>3̅<i>m</i>