An in-situ electron microscopy study of dual ion-beam irradiated xenotime-type ErPO₄

Rare-earth phosphates adopting the xenotime (REPO₄; RE = Tb − Lu & Y, Sc) structure are proposed as a potential matrix for the confinement of minor actinides. Minor actinides (e.g., Np, Am, Cm) undergo a radioactive decay process in which high-energy recoil atom (70–100 keV) and energetic alpha particles (4.5–5.8 MeV) are produced. In this study, the impact of these energetic decay products on the structure of xenotime-type ErPO₄ has been investigated via high energy dual ion-beam irradiation of ErPO₄ ceramics. Au²⁺ (1.5 MeV) and He⁺ (160 keV) ions were used to simulate the effects of recoil atom and α-particles, respectively. Multiple experiments were carried out in which the Au²⁺ and He⁺ ions with varying ion-fluences (ions/cm²) and ion-flux (ions/cm²/s) were implanted sequentially (Au²⁺ followed by He⁺ irradiation) and simultaneously (Au²⁺ + He⁺ irradiation) into ErPO₄ ceramics. Sequential ion-irradiation experiments have shown that the xenotime structure was amorphized by Au²⁺ ions at a relatively lower ion-fluence (5 × 10¹³ ions/cm²) in comparison to the monazite structure. Upon irradiation of the amorphous ErPO₄ with He⁺ ions, recrystallization of the amorphous xenotime due to α-particles was not observed. However, simultaneous ion-irradiation experiments on ErPO₄ showed that the amorphization of the xenotime structure was prevented upon deposition of higher amounts of electronic energy (E_electronic) in the lamella. Likewise monazite samples, the α-healing mechanism was also experimentally demonstrated in synthetic xenotime samples.