Formation of Fully Stoichiometric, Oxidation-State Pure Neptunium and Plutonium Dioxides from Molecular Precursors

Abstract

Amidate-based ligands (N-(tert-butyl)isobutyramide, ITA) bind κ² to form homoleptic, 8-coordinate complexes with tetravalent ²³⁷Np (Np(ITA)₄, 1-Np) and ²⁴²Pu (Pu(ITA)₄, 1-Pu). These compounds complete an isostructural series from Th, U-Pu and allow for the direct comparison between many of the early actinides with stable tetravalent oxidation states by nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction (SCXRD). The molecular precursors are subjected to controlled thermolysis under mild conditions with the exclusion of exogenous air and moisture, facilitating the removal of the volatile organic ligands and ligand byproducts. The preformed metal-oxygen bond in the precursor, as well as the metal oxidation state, are maintained through the decomposition, forming fully stoichiometric, oxidation-state pure NpO₂ and PuO₂. Powder X-ray diffraction (PXRD), scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDS) elemental mapping supported the evaluation of these high-purity materials. This chemistry is applicable to a wide range of metals, including actinides, with accessible tetravalent oxidation states, and provides a consistent route to analytical standards of importance to the field of nuclear nonproliferation, forensics, and fundamental studies.

Figure 1

Selected methods for the preparation of actinide dioxides (An = actinide) by (a) firing metal (often stabilized by Ga or Se) in humid air, adapted with permission from ref, Copyright Elsevier 2011, (b) laser-induced heating of actinide oxide powder in a pressurized vessel, and (c) precipitation of actinide oxalate followed by calcination.^,, (d) This work.

Figure 2

Synthetic route to 1-Np (Np(ITA)₄) and 1-Pu (Pu(ITA)₄), and their thermolysis products 2-Np (NpO₂) and 2-Pu (PuO₂).

Figure 3

Solution phase UV–vis–NIR stacked plot of 1-Np (top, red) and 1-Pu (bottom, blue) in Et₂O.

Figure 4

Structures of 1-Np (top) and 1-Pu (bottom). Both structures are represented with 50% probability thermal ellipsoids at 100 K and have hydrogen atoms omitted for clarity.

Figure 5

High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images of NpO₂ (2a-Np, a) and PuO₂ (2a-Pu, e) were prepared from 1-Np and 1-Pu at 270 °C, respectively. Energy dispersive X-ray spectroscopy (EDS) elemental maps of oxygen (b and f) or actinide (Np, c and Pu, g) with the corresponding spectra (Np, d and Pu, h). Note, Au, Ti, Fe, Cu and C are all present in the tomography tip, specimen holder, and TEM grids.

Figure 6

Powder X-ray diffraction patterns (PXRD) collected with Cu Kα radiation at room temperature for 2b-Np (top, red, lattice constant = 5.431(5) Å) compared to calculated NpO₂ (top, black, ICSD: 647175), and 2b-Pu (bottom, blue, lattice constant = 5.395(5) Å) compared to calculated PuO₂ (bottom, black, ICSD: 186183)..