Poster Presentation 24th Australian Conference on Microscopy and Microanalysis 2016

Direct mapping of Li-enabled octahedral tilt ordering and associated strain in nanostructured perovskites (#305)

Ye Zhu 1 , Ray L. Withers 2 , Laure Bourgeois 1 3 , Christian Dwyer 4 , Joanne Etheridge 1 3
  1. Department of Materials Science and Engineering, Monash University, Clayton, VIC, Australia
  2. Research School of Chemistry, College of Physical and Mathematical Sciences, The Australian National University, Canberra, ACT, Autralia
  3. Monash Centre for Electron Microscopy (MCEM), Monash University, Clayton, VIC, Autralia
  4. Department of Physics, Arizona State University, Tempe, Arizona, USA

Nanostructures with periodic phase separation hold great promise for creating 2D and 3D superlattices with extraordinary functionalities. Understanding the mechanism(s) driving the superlattice formation demands the underlying structure information. However, nanoscale structural modulations/fluctuations 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 enable 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 octahedral.

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 the 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. Amazingly, we observe a reversible annihilation/reconstruction of the tilt modulation correlated with delithiation/lithiation process, suggesting the structure transformation 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.

  1. Y. Zhu, R. L. Withers, L. Bourgeois, C. Dwyer & J. Etheridge Nature Materials published online (2015). doi:10.1038/nmat4390
  2. A. M. Abakumov, R. Erni, A. A. Tsirlin, M. D. Rossell, D. Batuk, G. Nénert & G. V. Tendeloo Chem. Mater. 25, 2670–2683 (2013).