The Ion Irradiation Effects On δ-phase And Nanocrystalline Pyrochlore

δ-phase and pyrochlore, which possess derivative structure of the basic structure fluorite, have attracted much attention because of their potential application in immobilization for minor actinides or high-level radioactive waste. Thus, the radiation tolerance of δ-phase and pyrochlore is the one of the most significate properties to be considered. Radiation tolerance of a material is dependent on its ability to resist the radiation-induced damage, such as defect clustering, swelling, phase transformation and amorphization. Here, the studies described in this project examine radiation damage behavior in δ-phase and pyrochlore of varying composition. The investigations discussed in this thesis address two objectives:(1) to study microstructural evolution in polycrystalline δ-phase A4Hf3O12(A = Lu, Sc) exposed to different ion beam radiation, and(2) to study the effect of grain size on the irradiation response of pyrochlore A2Ti2O7(A = Gd, Ho and Lu).Polycrystalline δ-phase A4Hf3O12(A = Lu, Sc) was irradiated with varying ion beam to study their radiation response. Different experimental irradiation conditions were used in these experiments including heavy ion 6 Me V Xe26+, semi-heavy ion 600 ke V Kr3+, and light ion 400 ke V Ne2+. Ion irradiation-induced microstructural evolution was examined by various techniques including grazing incidence X-ray diffraction(GIXRD) and transmission electron microscopy(TEM). Both in d-Lu4Hf3O12 and d-Sc4Hf3O12, a phase transformation from ordered rhombohedral to disordered fluorite(order-to-disorder) was observed. Furthermore, ion species “spectrum effect’’ was discovered in this studies: the threshold dose of light ions was found to be ovservably lower than the threshold dose to produce order-to-disorder transformation by using heavy ions. This suggest that light ions are more efficient than heavy ions in producing the retained defects that responsible for the O-D transformation. The theoretical calculations show that the O-D transformation of d-Lu4Hf3O12 was attributed to the anion oxygen Frenkel pair defect, while both anion Frenkel defects and cation antisite defects contribute to irradiation-induced O-D phase transformation in the d-Sc4Hf3O12. It is also observed that the O-D transformation is easier to occur in the d-Lu4Hf3O12 than d-Sc4Hf3O12 under the same ion irradiation conditions. We concluded that the δ-phase compounds with similar cation radius are more radiation resistant to the phase transformation.On the other hand, irradiation response of nanocrystalline A2Ti2O7(A = Gd, Ho and Lu) pyrochlore powders with grain sizes of 20-30 nm was investigated by 1 Me V Kr2+ ion bombardment. Different irradiation temperatures were used, from room temperature(293 K) up to a relatively high temperature(600 K). Here, in situ transmission electron microscopy(TEM) revealed that the critical amorphization fluence increases with the sequence of the nanocrystalline Gd2Ti2O7, Ho2Ti2O7 and Lu2Ti2O7, which exhibits the same trend as in coarse-grained titanate pyrochlores. Meanwhile, we found that the critical amorphization fluence for each nanocrystalline compound at room temperature was greater than that for their coarse-grained counterparts, indicating an enhanced amorphization resistance. The effect of temperature on the irradiation response of one of these compounds, nanocrystalline Lu2Ti2O7, was further examined by performing ion irradiation at an elevated temperature range of 480 to 600 K. The critical amorphization temperature(Tc) was found to be noticeably higher in nanocrystalline Lu2Ti2O7(610 K) than its coarse-grained counterpart(480 K), revealing that nanocrystalline Lu2Ti2O7 is less resistant to amorphization compared to its coarsegrained phase under high temperatures. We interpret these results with the aid of atomistic simulations. Molecular statics calculations find that cation antisite defects are less energetically costly to form near surfaces than in the bulk, suggesting that the nanocrystalline form of these materials is generally less susceptible to amorphization than coarse-grained counterparts at low temperatures where defect kinetics are negligible. In contrast, at high temperatures, the annealing efficiency of antisite defects by cation interstitials is significantly reduced due to the sink properties of the surfaces in the nanocrystalline pyrochlore, which contributes to the observed higher amorphization temperature in the nano-grained phase than in coarse-grained counterpart. Together, these results provide new insight into the behavior of nanocrystalline materials under irradiation.