Size dependence of radiation-induced amorphization and recrystallization of synthetic nanostructured CePO_4 monazite

摘要

Monazite, CePO_4, is considered an important phosphate-type structure for the incorporation and disposal of actinides. Nanocrystal-line monazite with particle sizes ranging from 20 nm to greater than 100 nm was synthesized. The displacive and ionizing effects of radiation were investigated, separately and simultaneously, for 1 MeV Kr~(2+) and 200 keV electron irradiations. In situ transmission electron microscopy observations indicated that CePO_4 nanoparticles can be readily amorphized by 1 MeV Kr~(2+) irradiation. Nanostructured monazite displays greater critical amorphization doses and lower critical temperatures than that of bulk natural monazite, suggesting enhanced amorphization tolerance. A strong size dependence on radiation-induced amorphization was observed in which the smaller-sized particles (20 nm) are less resistant to amorphization compared with larger-sized particles (40 nm). The excess surface energy of nanostructured materials, as suggested by the larger surface area upon the reduction of particle size, may alter the energy difference between amorphous and crystalline phases, thus affecting the radiation stability. With 200 keV electron-beam irradiation, CePO_4 previously amorphized by 1 MeV Kr~(2+) experienced an ionizing-radiation-induced recrystallization. A greater recrystallization rate was observed for the smaller-sized particles. Under simultaneous electron and displacive ion irradiations, CePO_4 displayed greater tolerance against amorphization, probably as a result of radiation-induced recovery of displacive damage by the ionizing radiation. The strong size dependence of displacive radiation-induced amorphization and ionizing-radiation-enhanced recrystallization processes for nanostructured CePO_4 implies that an optimized size regime may exist in which nanostructured materials are more tolerant of radiation-induced amorphization.