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Exclusive ferroelectric microstructure revealed for the first time

A team of researchers observed and reported for the first time the unique microstructure of a new ferroelectric material, enabling the development of lead-free piezoelectric materials for safer electronics, sensors and energy storage for human use. This work was conducted by Penn State’s Alem Group and in collaboration with research groups at Rutgers University and the University of California, Merced.

Ferroelectrics are a class of materials that demonstrate spontaneous electrical polarization when an external electrical charge is applied. This causes spontaneous electrical polarization when the positive and negative charges in the materials go to different poles. These materials also have piezoelectric properties, which means that the material generates an electrical charge under an applied mechanical force.

This allows these materials to produce electricity from energy such as heat, motion, or even noise that could otherwise be wasted. Therefore, they have the potential for alternatives to carbon-based energy, such as harvesting energy from waste heat. Additionally, ferroelectric materials are particularly useful for data storage and memory as they can remain in a polarized state without additional power, making them attractive for data storage and energy-saving electronics. They are also widely used in advantageous applications such as switches, major medical devices such as heart rate and ultrasound monitors, energy storage systems and actuators.

However, the more powerful piezoelectric materials contain lead, which is a big problem since lead is toxic to humans and animals.

“We would like to design a piezoelectric material that does not exhibit the disadvantages of current materials,” said Nasim Alem, an associate professor of materials science and engineering at Penn State and corresponding author of the study. “And right now, lead in all of these materials is a big drawback because lead is dangerous. We hope our study will result in a suitable candidate for a better piezoelectric system.”

To develop a path towards such a lead-free material with strong piezoelectric properties, the research team worked with calcium manganate, Ca3Mn2O7 (CMO). CMO is a new improper hybrid ferroelectric material with some interesting properties.

“The design principle of this material is to combine the movement of the material’s small oxygen octahedra,” said Leixin Miao, a graduate student in materials science and first author of the study in Nature communications. “There are octahedra of oxygen atoms in the material that can tilt and rotate. The term ‘improper ferroelectric hybrid’ means that we combine the rotation and tilt of the octahedra to produce ferroelectricity. It is considered a ‘hybrid’ because it is the combination of two. movements of the octahedra that generate that polarization for ferroelectricity. It is considered an “improper” ferroelectric since polarization is generated as a side effect. “

There is also a unique feature of CMO’s microstructure that is a mystery to researchers.

“At room temperature, there are some polar and non-polar phases that coexist at room temperature in the crystal,” Miao said. “And those coexisting phases are thought to be correlated with negative thermal expansion behavior. It is known that a material normally expands when heated, but this shrinks. This is interesting, but we know very little about the structure, such as how the phases polar and non-polar coexist “.

To better understand this, the researchers used atomic-scale transmission electron microscopy.

“The reason we used electron microscopy is because with electron microscopy, we can use atomic-scale probes to see the exact atomic arrangement in the structure,” Miao said. “And it was very surprising to observe polar bilayer nanoregions in CMO crystals. To our knowledge, this is the first time that such microstructure has been imaged directly in layered perovskite materials.”

Before, according to the researchers, it had never been observed what happens to a material that goes through such a ferroelectric phase transition. But with electron microscopy, they could monitor the material and what was happening during the phase transition.

“We monitored the material, what is happening during the phase transition and we were able to probe atom by atom what kind of bond we have, what kind of structural distortions we have in the material and how it might change as a function of temperature,” Alem said. “And that explains very well some of the observations people have had with this material. For example, when they get the coefficient of thermal expansion, no one really knows where it came from. Basically, this was going down to the atomic level and understanding physics at scale. atomic, chemistry and even the dynamics of the phase transition, how it is changing “.

This in turn would enable the development of powerful lead-free piezoelectric materials.

“Scientists have been trying to find new pathways to discover lead-free ferroelectric materials for many beneficial applications,” Miao said. “The existence of polar nanoregions is believed to benefit piezoelectric properties and we have now shown that through defect engineering, we may be able to design new strong piezoelectric crystals that would eventually replace all lead-containing materials for ultrasonic applications or actuators “.

The characterization work that revealed these unprecedented processes in the material was carried out at the Materials Research Institute facilities in the Millennium Science Complex. This included experiments with multiple transmission electron microscopes (TEMs) that made it possible to see things never seen before.

Another benefit of the study was the free software developed by the EASY-STEM research team, which allows for easier processing of TEM image data. This could potentially reduce the time it takes to advance scientific research and shift it to practical application.

“The software has a graphical user interface that allows users to enter with mouse clicks, so people don’t have to be programming experts but they can still generate amazing analysis,” said Miao.

Along with Miao and Alem, other authors of the study include Penn State Parivash Moradifar, a graduate student at the time, and Ke Wang, a personnel scientist with MRI. From the University of California, authors include Kishwar-E Hasin, graduate student in computational materials science and simulation, and Elizabeth A. Nowadnick, assistant professor of materials science and engineering. Other authors include Oak Ridge National Laboratory Debangshu Mukherjee, research and development associate scientist, and Sang-Wook Cheong of Rutgers University, distinguished professor, Henry Rutgers Professor, Professor of the Board of Governors and director of the Center for Quantum Materials Synthesis.

The study was supported by the National Science Foundation.

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