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 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.