Insights on dramatic radial fluctuations in track formation by energetic ions

We report on unexpected dramatic radial variations in ion tracks formed by irradiation with energetic ions (2.3 GeV ²⁰⁸Pb) at a constant electronic energy-loss (∼42 keV/nm) in pyrochlore-structured Gd₂ TiZrO₇. Though previous studies have shown track formation and average track diameter measurements in the Gd₂Ti_xZr_(1-x)O₇ system, the present work clearly reveals the importance of the recrystallization process in ion track formation in this system, which leads to more morphological complexities in tracks than currently accepted behavior. The ion track profile is usually considered to be diametrically uniform for a constant value of electronic energy-loss. This study reveals the diameter variations to be as large as ∼40% within an extremely short incremental track length of ∼20 nm. Our molecular dynamics simulations show that these fluctuations in diameter of amorphous core and overall track diameter are attributed to the partial substitution of Ti atoms by Zr atoms, which have a large difference in ionic radii, on the B-site in pyrochlore lattice. This random distribution of Ti and Zr atoms leads to a local competition between amorphous phase formation (favored by Ti atoms) and defect-fluorite phase formation (favored by Zr atoms) during the recrystallization process and finally introduces large radial variations in track morphology.