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

α-Na₃NpO₄ and α-Na₃PuO₄ exhibit an orthorhombic structure (Z = 8), in space group Fmmm, with lattice parameters a = 13.352(2) Å, b = 9.629(2) Å, and c = 6.673(2) Å for the neptunium compound, and a = 13.302(2) Å, b = 9.634(2) Å, and c = 6.651(2) Å 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 ²³⁷Np Mössbauer 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.