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

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 similar to 1.0 mu s and the UO66- ions resided at the La3+ sites with a lifetime of similar to 9.0 mu s. 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 degrees 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 Cphase transformation, and structure-optical property correlation.