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

Abstract

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.