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

Abstract

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 I4/m, while Na5NpO6 adopts a monoclinic unit cell in the space group C2/m. 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.

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

“…The Na 4 AnO 5 (An = U, Np, Pu) composition is common to all three systems with a tetragonal structure, in space group I4/ m. 9,29,30 Heptavalent Na 5 NpO 6 and Na 5 PuO 6 are both monoclinic, in space group C2/m. 19,30 All of these ternary oxide phases show 6-fold-coordinated actinide cations as described in detail in the following sections. Moreover, the mean U−O, Np−O, and Pu−O distances in the AnO 6 octahedra decrease when the valence state of the actinide cation increases, which is expected from the size of the respective ionic radii, as reported in ref 26.…”

Structural investigation of Na3NpO4 and Na3PuO4 using X-ray diffraction and 237Np Mössbauer spectroscopy

“…Keller and coworkers pioneered the study of the interaction between minor actinides (Np, Am) and alkali metals. 8,9 As part of our program of research, we have recently revisited the structural properties of the Na-Np-O 10,11 and Na-Pu-O systems, 12 and confirmed the formation of the hexavalent and heptavalent compounds Na 2 NpO 4 , Na 4 AnO 5 , Na 2 An 2 O 7 , and Na 5 AnO 6 (An = Np, Pu). The present study focuses on the pentavalent composition Na 3 AnO 4 , as this product is more likely to form under the oxygen potential conditions of the reactor.…”

A New Look at the Structural and Magnetic Properties of Potassium Neptunate K2NpO4 Combining XRD, XANES Spectroscopy, and Low-Temperature Heat Capacity

“…The XANES spectrum of K 2 NpO 4 collected at the Np-L 3 edge is shown in Figure together with those of NpO 2 , Na 3 Np V O 4 , α-Na 2 Np VI O 4 , Na 4 Np VI O 5 , and Na 5 Np VII O 6 reference materials . The valence states of the sodium neptunates were confirmed by 237 Np Mössbauer spectroscopy from the values of their isomer shifts, − while the corresponding XANES spectra were reported in ref . The values of the measured inflection points and white lines are reported in Table .…”

“…4−7 Those studies have revealed intriguing magnetic properties for the α-Na 2 Np VI O 4 7 and Na 4 Np VI O 5 6 compositions and have stressed the need to re-evaluate the Np(VI) crystal-field ground state.…”

“…A number of studies have recently been reported on several sodium actinide phases because of their relevance for the safety assessment of sodium-cooled fast reactors (SFRs). − Those studies have revealed intriguing magnetic properties for the α-Na 2 Np VI O 4 and Na 4 Np VI O 5 compositions and have stressed the need to re-evaluate the Np­(VI) crystal-field ground state.…”