X-ray Diffraction, Mossbauer Spectroscopy, Magnetic Susceptibility, and Specific Heat Investigations of Na$_4$NpO$_5$ and Na$_5$NpO$_6$

Abstract : The hexavalent and heptavalent sodium neptunate compounds Na$_4$NpO$_5$ and Na$_5$NpO$_6$ have been investigated using X-ray powder diffraction, Mossbauer spectroscopy, magnetic susceptibility, and specific heat measurements. Na$_4$NpO$_5$ has tetragonal symmetry in the space group 14/m, while Na$_5$NpO$_6$ 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 Mossbauer 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 Mossbauer studies. The absence of magnetic ordering has been confirmed for Na$_4$NpO$_5$. 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.