Temperature dependence of amorphization for zirconolite and perovskite irradiated with 1 MeV krypton ions

A transmission electron microscope investigation was made of zirconolites and perovskites irradiated to amorphization with 1 MeV krypton ions using the HVEM-Tandem Facility at Argonne National Laboratory. Three specimens were examined - a prototype zirconolite CaZrTi₂O₇, a gadolinium doped zirconolite Ca_0.75Gd_0.50Zr_0.75Ti₂O₇ and a uranium doped zirconolite Ca_0.75U_0.50Zr_0.75Ti₂O₇. The critical amorphization dose D_c was determined at several temperatures between 20 K to 675 K. D_c was inversely proportional with temperature. For example, pure zirconolite requiring 10x the dose for amorphization at 475 K compared with gadolinium zirconolite. Using an Arrhenius plot, the activation energy E_a for annealing in these compounds was found to be 0.129 eV and 0.067 eV respectively. The greater ease of amorphization for the gadolinium sample is probably a reflection of this element's large cross section for interaction with heavy ions. Uranium zirconolite was very susceptible to damage and amorphized under 4 keV argon ions during the preparation of microscope specimens. In each sample, zirconolite coexisted with minor perovskite, reduced rutile (Magneli phases) and zirconia. These phases were more resistant to ion irradiation than zirconolite. Even for high gadolinium loadings, perovskite (Ca_0.8Gd_0.2TiO₃) was 3-4 times more stable to ion irradiation than the surrounding zirconolite crystals.