Micrometric drilling of (meta-)studtite square platelets formed by pseudomorphic conversion of UO₂ under...

Pseudomorphic transformations are related to chemical conversions of materials while conserving their shape and structural features. Structuring ceramic shapes this way can be used to tailor the physico-chemical properties of materials that can benefit particular applications. In the context of spent nuclear fuel storage interacting with radiolysis products, the sonochemical behavior of powdered UO₂ was investigated in dilute aqueous solutions saturated with Ar/(20 %)O₂ (20 °C). Optimized parameter settings enabled the complete conversion of UO₂ micrometric platelets into uranyl peroxide precipitates, referred to as (meta-)studtite [(UO₂(O₂)(H₂O)₂)xH₂O] with x = 2 or 4. While the most acidic conditions yielded elongated crystal shapes in agreement with a dissolution/reprecipitation mechanism, softer conditions allowed the pseudomorphic transformation of the platelet shape oxide suggesting a complex formation mechanism. For specific conditions, this unprecedented morphology was accompanied with the formation of a hole in the platelet center. Investigations revealed that the formation of the drilled polymorphs is related to a perfect blend of H⁺, in-situ generation of H₂O₂ and high-frequency ultrasound, and is most probably related to the sono-capillary effect. These insights pave the way for new sonochemical approaches dedicated to the preparation of material polymorphs tailoring specific structural properties.