New discovery challenges understanding of ferroelectric materials

A new discovery has turned previously accepted knowledge about ferroelectric materials on its head. Northwestern University researchers have found that a unique class of ferroelectric oxides responds in an unexpected way to stress.

“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 polarisation,” said 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 polarisation increases.”

Layered perovskites grown as a thin film initially react to strain in the same way as other ferroelectrics: their polarisation increases. However, Rondinelli’s research team found that if further strain is applied, the polarisation goes away completely.

“Based on everything we have known for the past two decades, this is completely unexpected,” said Rondinelli.

Ferroelectrics are found everywhere: in smartphones, watches and computers. Because they are so technologically useful, researchers have long been interested in creating new or improved ferroelectric materials — especially in two-dimensional geometries as thin films where they are readily integrated into electronic devices. Ferroelectricity is a property that occurs when a material exhibits a spontaneous electric polarisation, which arises from a shift of positive and negative charges in opposite directions.

Layered perovskites have gained attention because they host functional physical properties like high-temperature superconductivity and support electrochemical or photocatalytic energy conversion processes. Their structures are also much more defect tolerant.

“You can’t strain the material too much because it might lose its functionality,” Rondinelli said. “But if you operate near where the polarisation turns on and off, you really have a switch. If you’re monitoring the polarisation 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.”

The discovery was made using theoretical materials tools and quantum mechanical simulations. Rondinelli’s team is now working to validate the finding in the laboratory. After this, they will work to better understand how this new functionality could help or hinder ferroelectric applications.

Rondinelli says researchers will now 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 force and dielectric properties. It requires us to think more carefully, and I suspect there is much more to learn,” Rondinelli said.

The research was reported in Nature Materials. Xue-Zeng Lu, a PhD student in Rondinelli’s laboratory, was the paper’s first author.