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The role of intermediate phases in CeO 2 mesocrystal formation from aqueous Ce III solutions subjected to g-radiation was studied. Radiolytically formed hydroxyl radicals convert soluble Ce III into less soluble Ce IV .T ransmission electron microscopy( TEM) and X-rayd iffraction studies of samples from different stages of the process allowed the identification of several stages in CeO 2 mesocrystal evolution following the oxidation to Ce IV :(1) formation of hydrated Ce IV hydroxides,s erving as intermediates in the liquid-to-solid phase transformation;(2) CeO 2 primary particle growth inside the intermediate phase;( 3) alignment of the primary particles into "pre-mesocrystals" and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of "mineral bridges". Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures.

The role of intermediate phases in CeO2 mesocrystal formation from aqueous CeIII solutions subjected to γ‐radiation was studied. Radiolytically formed hydroxyl radicals convert soluble CeIII into less soluble CeIV. Transmission electron microscopy (TEM) and X‐ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO2 mesocrystal evolution following the oxidation to CeIV: (1) formation of hydrated CeIV hydroxides, serving as intermediates in the liquid‐to‐solid phase transformation; (2) CeO2 primary particle growth inside the intermediate phase; (3) alignment of the primary particles into “pre‐mesocrystals” and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of “mineral bridges”. Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures.