Multi-timescale investigation of radiation damage near TiO2 rutile grain boundaries

Abstract

To understand the interactions between defects and grain boundaries (GBs) in oxides, two atomistic modeling methods were used to examine the role of GBs in a model system, rutile TiO2, in modifying radiation-induced defect production and annealing. Molecular dynamics was used to investigate defect production near a symmetric tilt GB at both 300 K and 1000 K. The damage production is found to be sensitive to the initial distance of the primary knock-on atom from the GB. We find three distinct regimes in which GBs have different effects. Similar to GBs in metals, the GB absorbs more interstitials than vacancies at certain distances while this behavior of biased loading of interstitials diminishes at other distances. Further, we obtain the statistics of both interstitial and vacancy clusters produced in collision cascades in terms of their compositions at two temperatures. Perfectly stoichiometric defect clusters represent a small fraction of the total clusters produced. Moreover, a significant reduction in the number of interstitial clusters at 1000 K compared to 300 K is thought to be a consequence of enhanced migration of interstitials towards the GB. Finally, the kinetic properties of certain defect clusters were investigated with temperature accelerated dynamics, without any a priori assumptions of migration mechanisms. Small interstitial clusters become mobile at high temperatures while small vacancy clusters do not. Multiple migration pathways exist and are typically complex and non-intuitive. We use this kinetic information to explain experimental observations and predict their long-time migration behavior near GBs.