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 °C–600 °C. 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.