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

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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 = 121.14 Å). 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 ∼0.02 Å shorter than the U-F distance in [NH4]4UF8 and is a result of the actinide contraction. © 2016 Elsevier B.V. All rights reserved.