Synthesis and crystal structure characterisation of sodium neptunate compounds

Abstract 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 NpO 2 /Na 2 O ratios led to the following polycrystalline compounds: Na 2 Np 2 O 7 monoclinic (P12 1 1), α-Na 2 NpO 4 orthorhombic (Pbam), β-Na 2 NpO 4 orthorhombic (Pbca), β-Na 4 NpO 5 tetragonal (I4/mmm), Na 5 NpO 6 monoclinic (C2/m) and a cubic compound (Fm-3m) that could either be Na 3 NpO 4 or Na 4 NpO 4 . The crystal structures of the α-Na 2 NpO 4 and Na 2 Np 2 O 7 compounds were refined by Rietveld analysis. Evolution of the cell parameters of α-Na 2 NpO 4 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: α a  = 41.3 × 10 −6  K −1 , α b  = 35.0 × 10 −6  K −1 , α c  ∼ 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.