Structural and Morphological Influences on Neptunium Incorporation in Uranyl Molybdates

Abstract

The in situ incorporation of pentavalent neptunium has been studied in the structurally related uranyl molybdate frameworks (NH4)4[(UO2)5(MoO4)7](H2O)5 and (NH4)2[(UO2)6(MoO4)7](H2O)2 prepared under similar synthetic conditions. The presence of Np(V) was confirmed by UV–vis–NIR spectroscopy in the first compound, whereas Np(VI) was identified in the second based on the observation of a unit-cell contraction and the lack of a spectral signature for Np(V). The incorporation of neptunium does not affect the overall structure of the host compound based on the crystallographic unit-cell parameters. Neptunium appears to preferentially incorporate in the structure of (NH4)2[(UO2)6(MoO4)7](H2O)2 due to the formation of Np(VI) during synthesis, although higher total uptakes were observed in (NH4)4[(UO2)5(MoO4)7](H2O)5 due to a higher initial concentration of neptunium in solution despite maintaining the same ratio of U:Np.