Jun 14 2016
Until recently, researchers were under the presumption that they had almost figured out the behavior of ferroelectric materials.
James Rondinelli (Credit: Northwestern University's McCormick School of Engineering)
“The conventional wisdom is that you can put almost any material under mechanical stress, and provided the stress is coherently maintained, the material will become ferroelectric or exhibit an electrical polarization,” said James Rondinelli, assistant professor of materials science and engineering at Northwestern Engineering. “If you apply similar stresses to a compound that’s already ferroelectric, then its polarization increases.”
However, a theoretical discovery made by Rondinelli and his team opposes this well-established fact. The researchers discovered that when a unique group of ferroelectric oxides are compressed or stretched, the polarization vanishes completely and does not increase as anticipated.
Based on everything we have known for the past two decades, this is completely unexpected.
James Rondinelli, Assistant Professor of Materials Science and Engineering, Northwestern University
The research has been reported in Nature Materials, and is supported by the National Science Foundation. The paper’s first author is Xue-Zeng Lu, a PhD student in Rondinelli’s laboratory.
Ferroelectrics have scope in various applications such as watches, computers, and smart phones. As ferroelectric Materials have a wide range of technological applications, researchers are trying to create new and improved ones, specifically 2D geometry, so that they can be easily incorporated into electronic devices.
Ferroelectricity is a phenomenon that takes place when material possesses a spontaneous electric polarization that occurs due to shift of negative and positive charges in opposite directions.
When the class of oxides known as layered perovskites and grown as a thin film is exposed to strain, they behave like other ferroelectrics initially, and their polarization increases. However, when additional strain is applied, the polarization turns off completely.
Recently, layered perovskites have attracted a great deal of attention as they support electrochemical or photocatalytic energy conversion processes and have functional physical properties such as high-temperature super conductivity. Also, layered perovskites are more tolerant to defects. The discovery made by Rondinelli brings a new level of interest to these famous materials.
You can’t strain the material too much because it might lose its functionality, but if you operate near where the polarization turns on and off, you really have a switch. If you’re monitoring the polarization for a logic device or memory element, you can apply a small electric field to traverse this boundary and simultaneously read and write the on-and-off state.
James Rondinelli, Assistant Professor of Materials Science and Engineering, Northwestern University
This discovery was made by Rondinelli’s team using quantum mechanical simulations and theoretical materials tools. Further, the team has partnered with experimental collaborators to corroborate their findings in the laboratory. The subsequent step would be to understand how this new functionality could hinder or help ferroelectric applications.
Meanwhile, Rondinelli added that researchers would need to take extra care whenever they apply mechanical stress to layered perovskite ferroelectrics, because, if excess strain is applied, it can lead to dire consequences.
“This finding motivates us to recalibrate our intuition regarding what interactions are expected between mechanical forces and dielectric properties,” Rondinelli said. “It requires us to think more carefully, and I suspect there is much more to learn.”