Structural study of the ammonium octafluoroneptunate, [NH4]4NpF8

The [NH4]4NpF8 salt was prepared from the solid-state reaction of NpO2 with NH4HF2 and characterized by powder X-ray diffraction and X-ray absorption fine structure spectroscopy. The diffraction results confirm the compound to be isostructural to [NH4]4UF8 with the following lattice parameter (a=13.054(4)Å, b=6.681(2)Å, c=13.676(5)Å, β=121.14Å). For the first time, a neptunium fluoride complex has been characterized by XAFS spectroscopy. The energy position of the white line and inflection of the XANES spectra of [NH4]4NpF8 are consistent with the presence of Np(IV). Adjustment of the EXAFS spectra indicates that the coordination number (7.4±1.5) and the average Np–F distance (2.26(1)Å) are consistent with the presence of the NpF8 dodecahedron. The average Np–F distance is ∼0.02Å shorter than the U–F distance in [NH4]4UF8 and is a result of the actinide contraction.