Until recently, researchers thought they had the behaviors of ferroelectric materials mostly figured out. These are materials that exhibit a spontaneous electrical polarization, as result of positive and negative charges in the material moving in opposite directions.
"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," explains James Rondinelli, assistant professor of materials science and engineering at Northwestern University's McCormick School of Engineering. "If you apply similar stresses to a compound that's already ferroelectric, then its polarization increases."
Rondinelli and his team, however, have now made a theoretical discovery that flips this widely accepted fact on its head. They found that when a unique class of ferroelectric oxides are stretched or compressed, the polarization does not increase as expected. Instead, it goes away completely.
"Based on everything we have known for the past two decades," Rondinelli said, "this is completely unexpected."
Supported by the US National Science Foundation, this research is reported in a paper in Nature Materials. Xue-Zeng Lu, a PhD student in Rondinelli's laboratory, served as the paper's first author.
"This finding motivates us to recalibrate our intuition regarding what interactions are expected between mechanical forces and dielectric properties. It requires us to think more carefully, and I suspect there is much more to learn."James Rondinelli, Northwestern University
Ferroelectric materials are found everywhere: in smart phones, watches and computers. Because they are so technologically useful, researchers have long been interested in creating new or improved ferroelectric materials – especially as thin films that can be readily integrated into electronic devices.
Rondinelli and his team found that when strain is applied to a class of oxides called layered perovskites grown as thin films, they initially react the same way as other ferroelectrics, with an increase in polarization. But if further strain is applied, the polarization turns off completely.
Layered perovskites have received much attention recently because of their ability to host functional physical properties like high-temperature superconductivity and to support electrochemical or photocatalytic energy conversion processes. Their structures are also highly defect tolerant. This latest discovery should now attract even more attention to these popular materials.
"You can't strain the material too much because it might lose its functionality," Rondinelli said. "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."
Rondinelli and his colleagues made this discovery using software tools and quantum mechanical simulations, and are now working with experimental collaborators to validate the discovery in the laboratory. They are also looking to better understand how this new functionality could help or hinder ferroelectric applications.
In the meantime, Rondinelli said researchers will need to be careful when applying mechanical stress to layered perovskite ferroelectrics, as applying too much strain could have unintended 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."
This story is adapted from material from Northwestern University's McCormick School of Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.