Σ3 Twin Boundaries in Gd2Ti2O7 Pyrochlore: Pathways for Oxygen Migration

Abstract

Understanding the chemistry at twin boundaries (TB) is a well-recognized challenge, which could enable the capabilities to manipulate the functional properties in complex oxides. The study of this atomic imperfection becomes even more important, as the presence of twin boundaries has been widely observed in materials, regardless of the dimensionalities, due to the complexities in growth methods. In the present study, we provide atomic-scale insights into a Sigma 3(11 (1) over bar) < 1 (1) over bar0 > 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) over bar) with a 71 degrees angle between two symmetrically arranged crystals. We observe distortions (similar to 3 to 5% strain) in the atomic structure at the TB with an increase in GdGd (0.66 +/- 0.03 nm) and Ti-Ti (0.65 +/- 0.02 nm) bond lengths in the (1 (1) over bar0) plane, as compared to 0.63 nm in the ordered structure. Using atomistic modeling, we further calculate the oxygen migration barrier for vacancy hopping at 48f48f sites in the pyrochlore structure, which is the primary diffusion pathway for fast oxygen transport. The mean migration barrier is lowered by similar to 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 Sigma 3 twin boundaries. Overall, these results offer a critical understanding of the atomic arrangement at the twin boundaries in pyrochlores, leading to control of the interplay between defects and properties.