5 research outputs found
Structural Properties and Charge Distribution of the Sodium Uranium, Neptunium, and Plutonium Ternary Oxides: A Combined Xâray Diffraction and XANES Study
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
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>)ÂO<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
TriclinicâCubic Phase Transition and Negative Expansion in the Actinide IV (Th, U, Np, Pu) Diphosphates
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 triclinicâcubic
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 PâOâP
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 PâO<sub>b</sub>âP linkage remains bent in the cubic
form
Xâray Diffraction, MoÌssbauer 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>
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,
MoÌssbauer 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., NpÂ(VI) and NpÂ(VII),
respectively, have been confirmed by the isomer shift values of their
MoÌssbauer 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 MoÌssbauer
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 NpÂ(VI) phase