PHASE TRANSITIONS IN STUDTITE AND METASTUDTITE: RESOLVING 150 YEARS OF CONFLICTING RESULTS

Uranyl peroxide minerals are important materials in the nuclear fuel cycle. Despite nearly 150 years of research, conflicting literature reports about the thermal stability of uranyl peroxides coupled with poorly understood reaction kinetics have led to pressurization and explosion of uranium storage containers. To understand the influence of applied heating rate on phase transition temperatures, synthesized uranyl peroxide minerals studtite (UO₂O₂·4H₂O) and metastudtite (UO₂O₂·2H₂O) are systematically monitored via powder X-ray diffraction (PXRD), Raman spectroscopy, and non-isothermal thermogravimetric analysis across a set of heating rates (1, 2, 5, and 10 °C/min). A strong linear correlation between sample heating rate and phase transition temperature is observed from PXRD, Raman spectroscopy, and thermogravimetric analysis. Distinct kinetic mechanisms are shown to govern the phase transition as a function of heating rate for studtite, whereas a consistent kinetic model describes the transition from metastudtite to UO_x regardless of heating rate. Insight into mechanisms of phase transformation is provided, and significant water retention during dehydration from studtite to metastudtite is indicated by PXRD, Raman spectroscopy, and thermogravimetric analysis. Mixed phase uranium oxide products are produced by calcination of metastudtite with a likely formation mechanism involving conversion of some uranyl centers from hexagonal to pentagonal bipyramidal coordination units via structural destabilization induced by peroxide liberation.