Insights on dramatic radial fluctuations in track formation by energetic ions.

Ritesh Sachan,Yanwen Zhang,Matthew F Chisholm,Christina Trautmann,Maik Lang,Eva Zarkadoula,William J Weber

doi:10.1038/srep27196

Abstract

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 Gd2TiZrO7. Though previous studies have shown track formation and average track diameter measurements in the Gd2TixZr(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.

Highlights

Concentric defect-fluorite shell layer, the atomic-level details on the local variability in amorphous core diameter and shell-layer thickness have been elusive
Any correlative molecular dynamics (MD) simulations and theoretical modeling of ion track formation due to electronic energy loss of ions become difficult to validate in the absence of quantitative atomic resolution experimental details[19]
Since the electronic energy loss by ions in a material is somewhat stochastic in nature, irrespective of the target material, the morphological difference in track formation in Gd2TiZrO7 and Gd2Ti2O7 is primarily attributed to the random distribution of Ti and Zr atoms on the B-site of the Gd2TiZrO7 pyrochlore structure

Summary

Introduction

Concentric defect-fluorite shell layer, the atomic-level details on the local variability in amorphous core diameter and shell-layer thickness have been elusive. We performed nanoscale depth-dependent high angle annular dark field (HAADF) plan view imaging of ion tracks in a 5th order aberration corrected scanning transmission electron microscope (STEM), which provided the incoherent images and ability to three-dimensionally reconstruct an ion track to get morphological information with atomic resolution[20]. This method has an advantage over cross-sectional TEM imaging, which is limited to qualitative track morphology information. This work provides an improved understanding of the complex nature of energetic ion-lattice interactions and recrystallization process in Gd2TiZrO7 pyrochlore materials by an integrated study of correlative HAADF/STEM and MD simulations

Methods

Results

Conclusion