Structural investigation of Na3NpO4 and Na3PuO4 using X-ray diffraction and 237Np Mossbauer spectroscopy

alpha-Na3NpO4 and alpha-Na3PuO4 exhibit an orthorhombic structure (Z = 8), in space group Fmmm, with lattice parameters a = 13.352(2) angstrom, b = 9.629(2) angstrom, and c = 6.673(2) angstrom for the neptunium compound, and a = 13.302(2) angstrom, b = 9.634(2) angstrom, and c = 6.651(2) angstrom for the plutonium analogue. The corresponding structure has been solved ab initio as no structural analogue could be found in the literature. The pentavalent state of neptunium has moreover been confirmed by Np-237 Mssbauer spectroscopy, and the local structural properties inferred from the X-ray Rietveld refinement have been related to the fitted quadrupole coupling constant and asymmetry parameters. The existence of a low temperature metastable m phase of Na3NpO4 and Na3PuO4, of the NaCl type, has also been suggested.