Kavli Affiliate: David A. Muller
| First 5 Authors: Harikrishnan K. P., Ruijuan Xu, Kinnary Patel, Kevin J. Crust, Aarushi Khandelwal
| Summary:
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. We
establish limits on film thickness for interfacial octahedral rotation
engineering and directly visualize an incommensurate octahedral rotation
pattern, arising from the flat dispersion of the associated phonon mode.
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