Electron
microscopy, conducted as part of the Shared Research Equipment (ShaRE)
User Program at the Department of Energy’s Oak Ridge National
Laboratory, has led to a new theory to explain intriguing properties in a
material with potential applications in capacitors and actuators.
A
research team led by ORNL’s Albina Borisevich examined thin films of
bismuth samarium ferrite, known as BSFO, which exhibits unusual physical
properties near its transition from one phase to another. BSFO holds
potential as a lead-free substitute for lead zirconium titanate (PZT), a
similar material currently used in dozens of technologies from sensors
to ultrasound machines.
Materials
such as BSFO and PZT are often called “materials on the brink” in
reference to their enigmatic behavior, which is closely tied to the
transition between two different phases. These phases are characterized
by structural changes in the material that produce different electrical
properties.
“The
best properties of the material are found at this transition,”
Borisevich said. “However, there has been a lot of discussion about what
exactly happens that leads to an enhancement of the material’s
properties.”
Using
scanning transmission electron microscopy, the team mapped the position
of atoms in BSFO films to find what happens to the local structure at
the transition between ferroelectric and antiferroelectric phases. The
team’s results are published in Nature Communications.
“We
discovered that neither of the two dominant theories could describe the
observed behavior at the atomic scale,” Borisevich said.
Some
theorists have proposed that the material forms a nanocomposite at the
transition. In this case, the energy of the boundaries between phases
would have to approach zero, but Borisevich’s team found experimentally
something entirely different: the boundary’s energy was instead
effectively negative.
“When
the energy of boundary is negative, it means that the system wants to
have as many boundaries as possible, but with atom sizes being finite,
you can’t increase it to infinity,” Borisevich said. “So you have to
stop at some short-period modulated structure, which is what happens
here.”
Based
on its observations, the team concluded that the mechanism behind the
observed behavior was linked to a relatively weak interaction called
flexoelectricity.
“Flexoelectricity
means that you bend a material and it polarizes,” said ORNL coauthor
Sergei Kalinin. “It’s a property present in most ferroelectrics. The
effect itself is not necessarily very strong on macroscopic scales, but
with the right conditions, which are realized in nanoscale systems, it
can produce very interesting consequences.”
Borisevich
adds that the team’s approach can be used to investigate a variety of
systems with similar phase boundaries, and she emphasizes the importance
of mapping out materials at the atomic scale.
“In
this particular case, electron microscopy is the only way to look at
very local changes because this material is a periodic structure,” she
said. “The decisive atomic-scale information had been missing from the
discussion.”
Researchers
include National Academy of Sciences of Ukraine’s Eugene Eliseev and
Anna Morozovska; University of New South Wales’s Ching-Jung Cheng and
Valanoor Nagarajan; National Chiao Tung University’s Jiunn-Yuan Lin and
Ying-Hao Chu; and University of Maryland’s Daisuke Kan and Ichiro
Takeuchi.
Source: Oak Ridge National Laboratory
