Synthesis and crystal structure characterisation of sodium neptunate compounds

The present work reports studies of the chemical reactions between neptunium dioxide and sodium oxide either in the presence of oxygen or inert gas (Ar), leading to compounds with hexavalent, heptavalent or pentavalent/tetravalent neptunium, respectively. Solid state synthesis with different NpO2/Na2O ratios led to the following polycrystalline compounds: Na2Np2O7 monoclinic (P1211), α-Na2NpO4 orthorhombic (Pbam), β-Na2NpO4 orthorhombic (Pbca), β-Na4NpO5 tetragonal (I4/mmm), Na5NpO6 monoclinic (C2/m) and a cubic compound (Fm-3m) that could either be Na3NpO4 or Na4NpO4. The crystal structures of the α-Na2NpO4 and Na2Np2O7 compounds were refined by Rietveld analysis. Evolution of the cell parameters of α-Na2NpO4 was also followed as a function of temperature up to 1273 K by X-ray diffraction. The corresponding linear thermal expansion coefficients along the different axis were determined: αa = 41.3 × 10−6 K−1, αb = 35.0 × 10−6 K−1, αc ∼ 0 K−1. From the high temperature X-ray diffraction experiment it was also possible to evidence formation of diverse phases at different temperatures and to review parts of the Na-Np-O system.