Self-assembled nanostructures are promising for creating 2D and 3D superlattices with exceptional functionalities. Understanding the mechanisms driving the superlattice formation demands the underlying structural information. However, nanoscale structural modulations intrinsic to these superlattices are difficult to be characterized by conventional diffraction-based structure determination. A real-space, direct imaging method is necessary to probe the local structure characteristics, providing essential information for theoretical understanding and subsequent design of structure-property correlations.
Using the aberration-corrected scanning transmission electron microscopy (STEM), we developed an optimized atomic-level bright-field (BF) condition to image the oxygen octahedra in perovskite oxides. We used multislice calculations to determine detector collection angles that allow oxygen octahedra to be imaged sensitively and robustly over large specimen thicknesses. These calculations also provided a calibration by which the octahedral-tilt angle can be measured quantitatively from the image of each octahedron.
Applying this real-space octahedral-tilt mapping on Li0.5–3xNd0.5+xTiO3, a promising solid electrolyte in Li-ion batteries, we directly revealed an unconventional superlattices with 2D modulated octahedral tilting. A mathematical description of the octahedral-tilt modulation was derived based on the quantitative tilt maps, which explicitly identified the high-order harmonic character of the modulation. Using simultaneous annular-dark-field (ADF) imaging, we also mapped the lattice parameters unit-cell by unit-cell, uncovering highly-localized strain associated with the tilt modulation. Furthermore, we demonstrate the tunability of the tilt modulation by changing Li stoichiometry. Fascinatingly, we observe a reversible annihilation/reconstruction of the tilt modulation correlated with delithiation/lithiation process, suggesting the structural transformation that is associated with Li-ion conduction in this promising Li-ion conductor.1
The above observations are largely inaccessible from conventional diffraction analysis,2 and lead to an unprecedented mechanically-coupled tilting competition model to explain the superlattice formation.1 Our real-space approach to quantify local octahedral structure and correlate it with strain can be applied to other advanced oxide systems.
This work was supported by the Australian Research Council (ARC) grants DP110104734 and DP150104483 and a Monash University IDR grant. The FEI Titan3 80-300 S/TEM at Monash Centre for Electron Microscopy was funded by the ARC Grant LE0454166.
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To cite this abstract:Ye Zhu, Ray Withers, Laure Bourgeois, Christian Dwyer, Joanne Etheridge; Direct mapping of Li-enabled octahedral tilt ordering and associated strain in nanostructured perovskites. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/direct-mapping-of-li-enabled-octahedral-tilt-ordering-and-associated-strain-in-nanostructured-perovskites/. Accessed: December 14, 2019
EMC Abstracts - https://emc-proceedings.com/abstract/direct-mapping-of-li-enabled-octahedral-tilt-ordering-and-associated-strain-in-nanostructured-perovskites/