Structural and Morphological Influences on Neptunium
Incorporation in Uranyl Molybdates
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Abstract
The in situ incorporation of pentavalent
neptunium has been studied
in the structurally related uranyl molybdate frameworks (NH<sub>4</sub>)<sub>4</sub>[(UO<sub>2</sub>)<sub>5</sub>(MoO<sub>4</sub>)<sub>7</sub>](H<sub>2</sub>O)<sub>5</sub> and (NH<sub>4</sub>)<sub>2</sub>[(UO<sub>2</sub>)<sub>6</sub>(MoO<sub>4</sub>)<sub>7</sub>](H<sub>2</sub>O)<sub>2</sub> 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 (NH<sub>4</sub>)<sub>2</sub>[(UO<sub>2</sub>)<sub>6</sub>(MoO<sub>4</sub>)<sub>7</sub>](H<sub>2</sub>O)<sub>2</sub> due to the formation of Np(VI) during synthesis, although higher
total uptakes were observed in (NH<sub>4</sub>)<sub>4</sub>[(UO<sub>2</sub>)<sub>5</sub>(MoO<sub>4</sub>)<sub>7</sub>](H<sub>2</sub>O)<sub>5</sub> due to a higher initial concentration of neptunium in solution
despite maintaining the same ratio of U:Np
