  Domain walls are of increasing interest in ferroelectrics because of their
unique properties and potential applications in future nanoelectronics.
However, the thickness of ferroelastic domain walls remains elusive due to the
challenges in experimental characterization. Here, we determine the atomic
structure of ferroelastic domain walls and precisely measure the polarization
and domain wall thickness at picometer scale using annular bright field imaging
in an aberration-corrected scanning transmission electron microscope. We find
that the domain wall thickness in PbZr0.2Ti0.8O3 and PbTiO3 thin films is
typically about one-unit cell, across which the oxygen octahedron distortion
behavior is in excellent agreement with first principles calculations.
Remarkably, wider domain walls about two-unit cells in thickness are also
observed for those domains walls are coupled with dislocations and underwent a
compressive strain. These results suggest that the thickness of ferroelastic
domain walls highly depends on their atomic environments. This study can help
us to understand the past debatable experimental results and provide further
insights into control of domain walls via strain engineering for their possible
applications in nanoelectronics.
