Thermal Decomposition of Neptunyl Ammonium Nitrate: Mechanistic Insights and Structural Characterization of the Np2O5 Intermediate Phase

Abstract

Neptunium (Np) possesses a rich and unique chemistry that often diverges from other actinide elements yet remains relatively underexplored compared with the other light actinides. A resurgence of interest in Np has been spurred by the application of ²³⁷Np for plutonium-238 (²³⁸Pu) production for use in radioisotope thermoelectric generators (RTGs), necessitating evaluation of Np chemical reactions and materials. The work presented here studied the thermal decomposition of neptunyl ammonium nitrate (NH₄Np^VIO₂(NO₃)₃) for synthesis of neptunium dioxide (NpO₂), which is the target material used for production of ²³⁸Pu. Additionally, structural characterization of the intermediate solid Np pentoxide (Np₂O₅) was performed. Advanced solid-state characterization techniques, including simultaneous thermal analysis (STA), powder X-ray diffraction (pXRD), Raman spectroscopy, and density functional theory (DFT) modeling have been combined to study the reaction pathways. Analysis revealed that NH₄Np^VIO₂(NO₃)₃ thermally decomposes to a proposed neptunyl nitrate intermediate, followed by Np₂O₅ and finally NpO₂, all within the temperature range of 150℃–600℃. Further characterization of the pentoxide intermediate provided the first Raman spectra of pure-phase Np₂O₅ and associated DFT modeling confirmed Raman peak assignments for this phase. These findings provide mechanistic information to advance production of the critical radioisotope 238Pu and advance the state of knowledge on Np materials chemistry using modern characterization techniques.