ABSTRACT Plutonium (Pu), a prominent representative of the 5f‐block actinides, is a redox active element. Pu (III) ‐Pu (VI) are the four thermodynamically stable redox states in aqueous solution. For Pu (VI), hydrolysis reactions in NaCl solutions start in weakly acidic conditions (pH c ≈ 5) ; increasing the alkalinity typically yields supersaturated solutions that are metastable with respect to the formation of Pu (VI) ‐oxido‐hydroxide solid‐phases. We show that needle‐ and platelet‐like crystals of Pu (VI) form in strongly alkaline NaCl‐NaOH solutions. Detailed characterization reveals that both are built from surprisingly novel, spherical (PuO 2) 60 O 20 (OH) 120 40− cage clusters that are interlinked by sodium ions. The topology of the Pu 60 units resembles a truncated dodecahedron (Archimedean solid), representing a rather uncommon case in structural chemistry. Each cage contains 20 identical trimeric subunits that correspond closely to the trimeric Pu (VI) solution complex predominant under the crystallization conditions. This indicates that both solids form by a self‐assembly from the precursor solution species, highlighting the trimer as a hitherto not considered principal building block in actinide cage cluster chemistry. The present study also addresses an open issue dating back to one of the earliest studies on Pu (VI) hydrolysis from 1949, which first described the two crystal types without a clear structural interpretation.
Fellhauer et al. (2026) studied this question.