5 research outputs found

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

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    The charge distributions in α-Na<sub>2</sub>UO<sub>4</sub>, Na<sub>3</sub>NpO<sub>4</sub>, α-Na<sub>2</sub>NpO<sub>4</sub>, Na<sub>4</sub>NpO<sub>5</sub>, Na<sub>5</sub>NpO<sub>6</sub>, Na<sub>2</sub>PuO<sub>3</sub>, Na<sub>4</sub>PuO<sub>5</sub>, and Na<sub>5</sub>PuO<sub>6</sub> are investigated in this work using X-ray absorption near-edge structure (XANES) spectroscopy at the U-L<sub>3</sub>, Np-L<sub>3</sub>, and Pu-L<sub>3</sub> edges. In addition, a Rietveld refinement of monoclinic Na<sub>2</sub>PuO<sub>3</sub>, in space group <i>C</i>2/<i>c</i>, is reported for the first time, and the existence of the isostructural Na<sub>2</sub>NpO<sub>3</sub> 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

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    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 <sup>241</sup>Am 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<sub>0.80</sub>U<sub>0.12</sub>Np<sub>0.06</sub>Pu<sub>0.02</sub>)­O<sub>1.8</sub> has been successfully synthesized to prepare a 2.96 g pellet containing 2.13 g of <sup>241</sup>Am 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 <sup>241</sup>Am 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

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    The <i>An</i>P<sub>2</sub>O<sub>7</sub> diphosphates (<i>An</i> = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the <i>An</i><sup>IV</sup> cation and its suitability for the unusual octahedral environment. Synchrotron and X-ray diffraction, thermal analysis, Raman spectroscopy, and <sup>31</sup>P nuclear magnetic resonance reveal them as a new family of diphosphates which probably includes the recently studied CeP<sub>2</sub>O<sub>7</sub>. Although they adopt at high temperature the same cubic archetypal cell as the other known MP<sub>2</sub>O<sub>7</sub> diphosphates, they differ by a very faint triclinic distortion at room temperature that results from an ordering of the P<sub>2</sub>O<sub>7</sub> units, as shown using high-resolution synchrotron diffraction for UP<sub>2</sub>O<sub>7</sub>. The uncommon triclinic–cubic phase transition is first order, and its temperature is very sensitive to the ionic radius of <i>An</i><sup>IV</sup>. 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<sup>IV</sup> cations in which the two phenomena coincide. As shown by various approaches, the P–O<sub>b</sub>–P linkage remains bent in the cubic form

    X‑ray Diffraction, Mössbauer Spectroscopy, Magnetic Susceptibility, and Specific Heat Investigations of Na<sub>4</sub>NpO<sub>5</sub> and Na<sub>5</sub>NpO<sub>6</sub>

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    The hexavalent and heptavalent sodium neptunate compounds Na<sub>4</sub>NpO<sub>5</sub> and Na<sub>5</sub>NpO<sub>6</sub> have been investigated using X-ray powder diffraction, Mössbauer spectroscopy, magnetic susceptibility, and specific heat measurements. Na<sub>4</sub>NpO<sub>5</sub> has tetragonal symmetry in the space group <i>I</i>4/<i>m</i>, while Na<sub>5</sub>NpO<sub>6</sub> 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 Na<sub>4</sub>NpO<sub>5</sub>. 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
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