Investigating the impact of gamma radiation on structural and optical properties of Eu3+ doped rare-earth hafnate pyrochlore nanocrystals

A proposed method for treating nuclear high level radioactive (NHLW) is to encapsulate it in a matrix made of complex oxide materials, such as pyrochlores with the general formula A2B2O7. Rare-earth hafnate pyrochlores have the potential to advance the current methods of NHLW disposal due to their robust chemical stability in radioactive environments, their high thermal stability, and their natural structural compatibility with radionuclide species. In this study, RE2Hf2O7:Eu3+ (RE = Y, La, Pr, Gd, Er, Lu) nanoparticles synthesized by a molten salt method were exposed to highly energetic gamma-ray irradiation. La2Hf2O7:Eu3+ and Lu2Hf2O7:Eu3+ underwent the order pyrochlore-disorder fluorite structural phase transition after exposure to gamma-ray irradiation. There was a change in the O-Eu charge transfer band (CTB) position as a function of gamma-ray dose. There was no change in the local symmetry of the Eu3+ dopant in Y2Hf2O7, Gd2Hf2O7 and Lu2Hf2O7, but in La2Hf2O7:Eu3+ gamma-ray dose reduced the symmetry around the Eu3+. For Y2Hf2O7:Eu3+ and La2Hf2O7:Eu3+, the lifetime and emission intensity were found to degrade possibly due to creation of gamma ray-induced defects which provides non-radiative pathways. Regarding Gd2Hf2O7:Eu3+, the concentration of oxygen vacancy defects predominated over other defects leading to enhanced emission and lifetime after gamma-ray irradiation. This study is of utmost importance for the design of a robust rare earth hafnate pyrochlore to be used as a nuclear waste host or gamma-ray based scintillator material.