Ion irradiation-induced amorphization of six zirconolite compositions

Abstract

Zirconolite is an important phase in Synroc, a polyphase titanate ceramic, designed for the immobilization of high level waste from nuclear fuel reprocessing. Zirconolite is a principal host phase for actinides in Synroc. We have studied radiation effects of six zirconolite compositions: CaZrTi2O7, Ca0.8Ce0.2ZrTi1.8Al0.2O7, Ca0.85Ce0.5Zr0.65Ti2O7, Ca0.5Nd0.5ZrTi1.5Al0.5O7, CaZrNb0.85Fe0.85Ti0.3O7 and CaZrNbFeO7. The samples have been irradiated by 1.0 MeV Kr+in a temperature range from 25 to 973 K and observed by in situ transmission electron microscopy. The radiation-induced crystalline-to-amorphous transformation was studied by electron diffraction and high-resolution electron microscopy. Concurrent with amorphization, monoclinic zirconolite has been found to transform to a fluorite sublattice through cation disordering during irradiation. All the zirconolites amorphized after a similar dose (∼2×1015−3.9×1015ions/cm2) at room temperature. The amorphization dose increased at elevated temperatures with varying rates of increase for each phase. The critical temperatures for amorphization were 550 K for CaZrNbFeO7, 590 K for CaZrNb0.85Fe0.85Ti0.3O7, 640 K for CaZrTi2O7, 900 K for Ca0.8Ce0.2ZrTi1.8Al0.2O7, 1000 K for Ca0.85Ce0.5Zr0.65Ti2O7, and 1020 K for and Ca0.5Nd0.5ZrTi1.5Al0.5O7. The trend of the critical temperatures indicates that decreasing calcium content increases susceptibility to amorphization. The role of calcium in the radiation-induced amorphous structure is considered to be that of a network-modifier of the aperiodic structure formed by the polyhedra of high-valence cations.