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