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Keeney et al. Microstructures 2023;3:2023041 https://dx.doi.org/10.20517/microstructures.2023.41 Page 11 of 15

Figure 4. Representative topography and lateral (lat) DART-PFM phase and amplitude (amp) images of 5.6 nm (1 u.c.) B6TFMO on
NGO (001) as a function of AFM-based nano-machined depth. The scanning direction of the cantilever scanning direction was parallel
with the [100] substrate axis.

Crystal symmetry of the Aurivillius phases dictates that the major in-plane spontaneous polarization
component lies entirely along the a axis (with a minor polarization along the out-of-plane c-axis in odd-
layered structures) . Therefore, there must be another physical origination for the experimentally observed
[43]
45° striped domains. a-b crystal twinning is a common occurrence in layered materials such as the layered
high temperature superconductors and Aurivillius phases . The lack of uniaxial polarization in this work
[54]
[53]
indicates that crystal twinning is present in the B6TFMO thin films. The crystal twinning would originate
from the orthorhombic crystal structure of the underlying (001) NGO substrate, which presents an
anisotropic template for growth based on a pseudo-cubic unit cell . The a axis of the pseudo-cubic cell lies
[53]
along the ab diagonals of the orthorhombic NGO crystal structure, and a pseudo-cubic lattice constant of
3.858 Å is presented for NGO. This provides a close lattice match to the a ’ of B6TFMO (3.89 Å) to grow
p