Materials for Energy & Utilities: ∑3 Twin Boundaries in Gd2Ti2O7 as pathways for fast oxygen migration

One of the critical requirements for efficient energy conversion and storage applications is to explore new materials for fast oxygen transport. This opens a significant challenge to the materials science community to investigate the atomic level phenomena and understand the correlation between atomic structure and ionic conduction. Pyrochlore oxides are the well-suited candidate for fast oxygen transport due to one inherent vacant oxygen site in the unit atomic structure. Recently, the introduction of twin boundaries in complex oxide materials, including pyrochlore oxides, has attracted a lot of attention due to the potential of twin boundaries as fast oxygen transport pathways. However, understanding the atomic-level chemistry at twin boundaries (TB) is a well-recognized challenge to enhance the fundamental understanding of this correlation. The objective of this study is to provide atomic-scale insights into a ∑3(11-1) ⟨1-10⟩ twin boundary present in pyrochlore-structured Gd₂Ti₂O₇ using atomic resolution electron microscopy and atomistic modeling. The formation of the observed TB occurs along (11-1) with a 71° angle between two symmetrically arranged crystals. We observe distortions (∼3 to 5% strain) in the atomic structure at the TB with an increase in Gd−Gd (0.66 ± 0.03 nm) and Ti−Ti (0.65 ± 0.02 nm) bond lengths in the (1-10) plane, as compared to 0.63 nm in the ordered structure. The oxygen migration barrier for vacancy hopping at 48f−48f sites, which is the primary diffusion pathway for fast oxygen transport in the pyrochlore structure was further analyzed using atomistic modeling. The mean migration barrier is lowered by ∼25% to 0.9 eV at the TB as compared to 1.23 eV in the bulk, suggesting the ease in oxygen transport through the ∑3 twin boundaries, which is expected to provide an efficient diffusion pathway.