| Popis: |
Oxide ceramics are the candidate materials for encapsulating minor actinides. For this application, knowledge of structural changes and stability under radiation environments is required. Oxygen-deficient fluorite structural derivatives (M1−xM’xO2−x, where M and M’ are aliovalent metal cations and O is the anion), which contain oxygen vacancies to provide a charge compensation mechanism that satisfies the electroneutrality, have attracted the attention for their radiation effects because oxide ceramics with a fluorite type structure are known to exhibit excellent radiation resistance. In the present study, we examined the structural changes of swift heavy ion irradiated δ-Sc4Hf3O12. Polycrystalline sintered pellets of δ-Sc4Hf3O12 were irradiated at room temperature with 92 MeV xenon (Xe26+) ions to fluences ranging from 3×1012 to 1×1e14 /cm2. The specimens were characterized by grazing incidence x-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy. We found an ordered δ-phase transforms to a disordered fluorite phase with increasing ion fluence. In addition, a new ordered phase with a short-range structure different from the δ-type was found from the surface to a depth of ∼4.5 μm in the specimen irradiated to a fluence of 1 × 1e14/cm2. Electron diffraction patterns were identical to those of the bixbyite structure. This means the following structural changes were induced by irradiation: δ-type [space group: R3 ̅ (No. 148)] → fluorite [Fm 3 ̅m (No. 225)] → bixbyite [Ia3 ̅ (No. 206)]. This structural change is anomalous because it is the opposite process of the usual irradiation-induced structural change, the order-to-disorder phase transformation [1]. [1] M. Iwasaki et al., J. Appl. Phys. 132, 075901 (2022). |
