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Page 2 of 17 Cabral et al. Microstructures 2023;3:2023040 https://dx.doi.org/10.20517/microstructures.2023.39

INTRODUCTION
Ferroic materials constitute a crucial category of materials that possess a variety of unique properties,
including ferroelectricity, ferromagnetism, and ferroelasticity, that are employed in numerous applications.
These applications encompass areas such as energy harvesting, sensors, medical imaging, and consumer
[1-5]
electronics . The properties and performance of these materials are intricately linked to their atomic-scale
structures and chemistries. In certain instances, even a slight modification in composition can profoundly
influence material performance. For instance, in both ceramics and single crystals, the piezoelectric
coefficient of the relaxor ferroelectric PMN-PT can be nearly doubled by substituting < 1 mol% samarium
(Sm) for lead (Pb) as demonstrated by Li et al. . Additionally, incorporating elements with varying valence
[6,7]
states, ionic radii, electronegativities, and polarizabilities holds great potential in bolstering the piezoelectric
and dielectric characteristics of ferroic materials, as demonstrated in the case of high-entropy alloys .
[8]
Understanding the correlation between chemical distribution and structure becomes pivotal in
comprehending the enhanced piezoelectric properties that arise in these materials. This understanding can
be harnessed to design and engineer the next generation of high-performance ferroic materials.

Advanced scanning transmission electron microscopy (STEM) is an exceptionally powerful tool that
enables direct visualization of atomic structure and chemistry in numerous materials. While electron
microscopes have long provided nanometer-scale resolutions, the introduction of aberration-correction
technology at the beginning of the 21st century has pushed the limits of resolution to sub-Angstrom length
scales [9-11] . This significant enhancement in imaging resolution, combined with improved accuracy and
precision in STEM imaging, has ushered in a new era of electron microscopy applications. By harnessing
the capabilities of an Angstrom-sized probe, it becomes possible to directly visualize atomic-scale chemistry
and structure. For instance, annular dark-field (ADF) STEM imaging employs the mass contrast (Z)
provided by the technique to identify individual dopant atoms within a bulk material [12,13] . These techniques
have further advanced, with electron ptychography achieving reported STEM resolutions as fine as 39 pm
and capable of resolving interstitial atoms in a matrix [14,15] . Moreover, imaging techniques in electron
microscopes can readily integrate with spectroscopic methods, such as energy-dispersive X-ray
spectroscopy (EDS) and electron energy loss spectroscopy (EELS), facilitating the examination of atomic-
scale chemistry, electronic structure, and even vibrational modes [16,17] .

STEM continues to be a vital tool for studies of ferroic and other functional materials. These applications
will continue to evolve with developments in electron optics, instrumentation, detectors, and in-situ
[19]
[18]
capabilities . Techniques, such as electron ptychography , 4D-STEM , and ultra-high energy resolution
[14]
EELS , have allowed for the characterization of parameters, such as polarization, chemical/structural
[17]
ordering, oxidation states, and electronic structure, at sub-nanometer length scales. Despite the
technological advancements, increased accessibility, and user-friendliness of electron microscopes, which
are now widely available in both industry and academia, a key challenge remains in bridging the gap
between researchers specializing in electron microscopy technique development and materials researchers.
Nonetheless, it is crucial for scientists in both domains to recognize the broad applicability of STEM for
characterizing multifunctional materials. This perspective aims to provide insights into the latest
developments in STEM instrumentation and techniques, emphasizing their broad utilization in ferroic
materials research. Additionally, opportunities for in-depth data analysis to address materials-related
questions will be discussed.

Basics of S/TEM
Aberration-correction technology has advanced the resolving power of conventional TEM and STEM from
the nanometer to sub-Angstrom scales. High-resolution TEM (HRTEM) and STEM differ in electron optics