On structure and phase transformation of uranium doped La2Hf2O7 nanoparticles as an efficient nuclear waste host

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The design and development of efficient and stable nuclear waste hosts has drawn intensive interest for long-lived lanthanides and actinides. A detailed investigation of their structure and potential structural evolution are crucial. In this study, we have synthesized lanthanum hafnate La2Hf2O7 nanoparticles (NPs) doped with uranium at different concentrations (0–10%) and investigated their structural transition. We have discovered that in our La2Hf2O7:U NPs, the uranium dopants are stabilized at both U4+ and U6+ oxidation states in which the U6+ oxidation state exists in octahedral uranate UO66− form. We also confirmed that the U4+ ions substituted the Hf4+ ions with a lifetime of ∼1.0 μs and the UO66− ions resided at the La3+ sites with a lifetime of ∼9.0 μs. More interestingly, the proportion of the U4+ ions in the La2Hf2O7:U NPs was higher than that of the UO66− ions at low doping level, but at the doping level higher than 2.5%, the fraction of the UO66− ions was greater than that of the U4+ ions. Furthermore, we studied the structural phase transformation from order pyrochlore to cotunnite of these La2Hf2O7:U NPs with increasing uranium doping level, and found that ordered pyrochlore phase favors the U4+ ions whereas disordered cotunnite phase favors the UO66− ions. We further used in situ Raman spectroscopy to confirm the reversible cotunnite to pyrochlore phase transformation of the La2Hf2O7:10%U NPs at 900 °C. Therefore, this work demonstrated the successful development of uranium doped La2Hf2O7 NPs and thorough characterization of the fundamental spectra of uranium ions, doping induced phase transformation, and structure–optical property correlation.

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Materials Chemistry Frontiers