Résumé
The solution stability of plutonium(IV) oxo-hydroxo clusters is a central issue in actinide chemistry, with broad implications for fundamental coordination chemistry, the nuclear fuel cycle, and environmental migration. In this work, we investigate the long-term evolution of the hexanuclear [Pu6O4(OH)4]12+ cluster stabilized by acetate ligands. Using ultraviolet–visible (UV–vis) absorption spectroscopy coupled with X-ray absorption (XAS) and small-angle X-ray scattering (SAXS) synchrotron techniques, we probed its structural transformation over nearly one year of aging. UV–vis and XAS analyses revealed a progressive loss of the molecular cluster signature accompanied by the emergence of low Pu–Pu coordination environments (CN = 5–6), characteristic of PuO2 colloidal nanoparticles. In contrast, SAXS measurements indicated a gradual increase in the apparent particle diameter of up to 10–15 nm, which is inconsistent with the formation of compact 2–3 nm PuO2 nanoparticles usually observed in aqueous solutions. The combined results most probably support a mechanism in which the hexameric cluster first converts into PuO2 primary colloids, which subsequently assemble into larger aggregates without crystallographic coalescence.