Modern insights into the mechanisms of neptunium oxalate decomposition

Neptunium oxalate (Np(C₂O₄)₂·6H₂O) is a historically relevant solid phase used in nuclear processing as a precursor for neptunium dioxide (NpO₂). Although Np oxalate has been synthesized and used for NpO₂ production for decades, the thermal decomposition mechanism of this phase remains poorly understood and has not been evaluated in over 30 years. Conflicting reports in historical literature suggest either a direct conversion from anhydrous oxalate to NpO₂ or a decomposition that includes the formation of Np carbonate or oxidized Np intermediate phases. In this work, we reexamine the decomposition pathway of Np(C₂O₄)₂·6H₂O using thermal analysis coupled with evolved gas analysis and temperature-dependent Raman spectroscopy to elucidate decomposition mechanisms and intermediate phases using modern analytical techniques. Thermal analysis revealed a three-stage decomposition process, including dehydration below 200 °C, oxalate breakdown between 170 and 370 °C, and NpO₂ formation by 500 °C. However, an unidentified plateau in the thermal data was observed during measurements. Raman spectroscopy confirmed the stages of decomposition, and in the analysis of potential intermediate phases, no carbonate phases or Np₂O₅ were identified. Raman data suggest that residual oxalate or nonstoichiometric oxide are present during decomposition before pure NpO₂ is formed. These findings clarify aspects of the Np oxalate decomposition mechanism and address longstanding discrepancies in the literature, with a specific focus on Np-specific materials chemistry.