∑3 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 ∑3(111̅ ) ⟨11̅ 0⟩ twin boundary present in pyrochlore-structured Gd 2 Ti 2 O 7 using atomic-resolution electron microscopy and atomistic modeling. The formation of the observed TB occurs along (111̅ ) 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 (11̅ 0) 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 48f−48f sites in the pyrochlore structure, which is the primary diffusion pathway for fast oxygen transport. 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. 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.

Probing Multiscale Disorder in Pyrochlore and Related Complex Oxides in the Transmission Electron Microscope: A Review

“…GPA has also been used to map strain at La 2 Zr 2 O 7 (LZO)/YSZ (111) interfaces, highlighting the presence of misfit dislocations that may impact the accumulation of radiation damage at such features ( Kaspar et al, 2017 ). This approach has also been used to demonstrate the effect of strain at twin boundaries in Gd 2 Ti 2 O 7 , showing a local reduction of the oxygen migration barrier compared with the bulk ( Gupta et al, 2020 ).…”

“…Heterointerface engineering of fluorites, perovskites and pyrochlores has been shown to control local oxygen diffusivity ( Schweiger et al, 2017 ), to affect local radiation response ( Spurgeon et al, 2020 ) and to produce novel topological phases ( Gallagher et al, 2016 ; Li et al, 2021 ). Other types of extended crystalline defects, such as dislocations ( Sheth et al, 2016 ; Shafieizadeh et al, 2018 ) and grain boundaries ( Bowman et al, 2015 ; Bowman et al, 2017 ; Perriot et al, 2017 ; Gupta et al, 2020 ; Syed et al, 2020 ; Tong et al, 2020 ) have been shown to behave as fast oxygen transport pathways in some cases. Even small, localized, atomic displacements such as local changes in bonding environment (e.g., coordination), polyhedral distortions (e.g., bond length or angle), polyhedral tilting and the adoption of short-range polyhedral configurations or ‘structural motifs’ ( Shamblin et al, 2016 ; Sun et al, 2016 ) can affect functional properties.…”

Structure, thermostability and magnetic properties of cubic Ce2−xTi2O7 pyrochlore obtained via sol–gel preparation

“…8 For instance, a typical pyrochlore Gd 2 Ti 2 O 7 demonstrates an insulating behavior, whereas Gd 2 Zr 2 O 7 is a good oxide-ion conductor. 9,10 Lanthanum-based pyrochlore/perovskite oxides have become one of the most investigated materials in the past 50 years. 11,12 It has been found that multiple A/B element substitutions may result in lattice distortions and structural transformations, especially the combination of A/B with different valences.…”

“…8 For instance, a typical pyrochlore Gd 2 Ti 2 O 7 demonstrates an insulating behavior, whereas Gd 2 Zr 2 O 7 is a good oxide-ion conductor. 9,10…”

Oxygen Diffusion in Ceramic Mixed Conducting La0.6Sr0.4Co0.2Fe0.8O3−δ : The Role of Grain and Twin Boundaries

“…All these interfaces can play a dominant role in overall device performance, however as lower temperature operation is increasingly of interest, the role played by grain boundaries is receiving more scrutiny. 2,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Hence an understanding of grain boundary transport in polycrystalline MEIC materials, both of cations 23 and anions (oxygen ions), is becoming necessary. Understanding cation transport is relevant to segregation and degradation effects over the very long timescales necessary for operation and during high temperature processing.…”