Insight into the effect of A-site cations on structural and optical properties of RE2Hf2O7:U nanoparticles

Abstract

Abstract A2B2O7 type pyrochlores have been recently proposed as a potential nuclear waste host due to their many interesting properties. To assess and understand the performance of these compounds as nuclear waste hosts, the speciation and structural investigations on actinide-doped RE2Hf2O7 are needed since both are imperative from their application perspective. In this work, we investigated the effect of uranium doping at different concentrations in the range of 0–10% on the structural and optical properties of RE2Hf2O7:U (RE = Y, Gd, Nd, and Lu) nanoparticles (NPs). The Y2Hf2O7 NPs exist in slightly disordered pyrochlore structure and the extent of disordering increases as a function of uranium doping while the structure reaches a cotunnite phase at 10.0% doping level. The Nd2Hf2O7 NPs also exist in a distorted pyrochlore structure and their distortion increases with increasing uranium doping inducing a phase transition into a disordered fluorite structure at 10.0% uranium doping. Both Gd2Hf2O7 and Lu2Hf2O7 NPs exist in a disordered fluorite structure and transforms into cotunnite structure at higher U concentrations (≥5.0%). Photoluminescence spectroscopy showed that uranium ions are stabilized in +6 oxidation state in all samples: in the form of uranate ion UO66− in the Y2Hf2O7, Nd2Hf2O7 and Lu2Hf2O7 NPs while in the form of uranyl ion UO22+ in the Gd2Hf2O7 NPs. Therefore, this work deepens the understanding of the behavior of uranium ions doped in different RE2Hf2O7 host matrices in the perspective of their application as nuclear waste hosts.