Electron ptychography reveals a ferroelectricity dominated by anion displacements

TL;DR

This study uses advanced electron ptychography to reveal that ferroelectricity in sodium niobate is primarily driven by anion displacements, providing detailed 3D structural insights beyond conventional microscopy.

Contribution

The paper introduces multislice electron ptychography to analyze ferroelectric structures, uncovering the dominance of anion displacements over cation distortions in sodium niobate.

Findings

Ferroelectricity is dominated by anion displacements.

Electron ptychography achieves sub-angstrom resolution.

Visualized random octahedral rotation patterns.

Abstract

Sodium niobate, a lead-free ferroic material, hosts delicately-balanced, competing order parameters, including ferroelectric states that can be stabilized by epitaxial strain. Here, we show that the resulting macroscopic ferroelectricity exhibits an unconventional microscopic structure using multislice electron ptychography. This technique overcomes multiple scattering artifacts limiting conventional electron microscopy, enabling both lateral spatial resolution beyond the diffraction limit and recovery of three-dimensional structural information. These imaging capabilities allow us to separate the ferroelectric interior of the sample from the relaxed surface structure and identify the soft phonon mode and related structural distortions with picometer precision. Unlike conventional ferroelectric perovskites, we find that the polar distortion in this material involves minimal distortions of the cation sublattices and is instead dominated by anion displacements relative to the niobium sublattice. We establish limits on film thickness for interfacial octahedral rotation engineering and directly visualize a random octahedral rotation pattern, arising from the flat dispersion of the associated phonon mode.