C60 and U ion irradiation of Gd2TixZr2-xO7 pyrochlore

Gd2TixZr2-xO7 (x = 0 to 2) pyrochlore was irradiated by 30 MeV C-60 clusters, which provide an extremely high ionizing energy density. High-resolution transmission electron microscopy revealed a complex ion-track structure in Gd2Ti2O7 and Gd2TiZrO7, consisting of an amorphous core and a shell of a disordered, defect-fluorite structure. As compared with the irradiation by 1.5 GeV U ions with the highest energy loss, the track structure is consistent with tracks created by monoatomic swift heavy ions, but the diameters (with the entire diameter of 17 nm for Gd2Ti2O7 and 15 nm for Gd2TiZrO7) are significantly larger due to the much smaller velocity and higher energy density of the C-60 ions. Ion tracks created by monoatomic ions are challenging to describe by HRTEM, as the boundary between disordered fluorite and pyrochlore matrix is less distinct. However, the C-60 irradiation shows a clearly resolved ion track with completely crystalline, disordered, defect-fluorite structure around an amorphous core. Based on the distinct boundaries of the track morphology, inelastic thermal-spike calculations were used to describe the track size and extract critical energy densities for the interpretation of the complex core-shell morphologies for the different pyrochlore compositions.