Researchers have demonstrated a new technique for precisely controlling phase boundaries in thin film materials by manipulating the thickness of those films – allowing them to engineer energy storage materials that do not rely on toxic elements. In proof-of-concept testing for the new technique, the researchers found that a nontoxic thin film has extremely promising dielectric properties, raising the possibility of creating new capacitor technologies that don’t rely on toxic materials.

The same material can have more than one crystalline structure, and phase boundaries are the areas in the material where those crystalline structures essentially coexist as the material transitions from one dominant crystalline structure into another. These phase boundaries are important because they can enhance specific characteristics of the material.

“For example, engineers use chemistry techniques to control the distribution of phase boundaries in some materials to make them better at storing charge,” says Ruijuan Xu, corresponding author of a paper on the new technique and an assistant professor of materials science and engineering at North Carolina State University. “However, many of these materials make use of toxic elements, such as lead. And it has been extremely difficult to manipulate the phase boundaries of nontoxic thin film materials with comparable characteristics because they make use of volatile elements, such as sodium.

“We’ve now developed a technique that allows us to control the distribution of phase boundaries in these nontoxic thin films without using chemical techniques,” Xu says. “Specifically, we’ve found that controlling the physical strain on a material has a profound effect on the distribution of phase boundaries within that material. And we can control the amount of strain by varying the thickness of the thin film – the thinner the film, the more strain the material is under.”

The researchers demonstrated the new technique using sodium niobate (NaNbO₃), which is a chemically simple end-member of the potassium sodium niobate (KNN) family, a class of lead-free materials that hold promise for use in ferroelectric applications. However, research on NaNbO₃ thin films has been limited because sodium is highly volatile, making it difficult to engineer the phase boundaries of the material using traditional chemistry techniques.

In proof-of-concept testing for their strain-driven technique, the researchers synthesized epitaxial crystalline NaNbO₃ thin films using pulsed laser deposition, precisely controlling the film thickness. They found there is a linear relationship between the thin film’s thickness and the distribution of two phases or crystalline structures – M_B and M_C – in the NaNbO₃ thin films. The thinner the film, the more M_C dominates.

“Essentially, you can control how much M_B versus M_C you have in the material,” Xu says. “And the relative amounts of M_B and M_C interact in complex ways that influence the phase boundaries in the thin film.”

In testing, the researchers were surprised to find that the NaNbO₃ thin film has attractive dielectric properties.

“We found that we could engineer the NaNbO₃ thin films so that their dielectric permittivity – or how much charge they can store – is comparable to, or better than, the dielectric permittivity of the best lead-based thin films,” Xu says. “This is extremely promising for engineering new capacitor technologies.

“We also found that we could control the extent to which the material’s dielectric properties are tunable,” Xu says. “Tunability refers to the ability to control how much charge the material stores by applying an electric field to the material. This property is critical for applications such as communication technologies.”

The researchers also note that, while the new technique was demonstrated using NaNbO₃, it could also be used for a range of other thin films.

“We’re particularly interested in using the technique to study a wide range of other lead-free systems, such as the KNN family, to explore their potential for next-generation lead-free dielectric and ferroelectric applications,” Xu says.

The paper, “Strain-induced lead-free morphotropic phase boundary,” is published open access in the journal Nature Communications. First author of the paper is Reza Ghanbari, a Ph.D. student at NC State. Other NC State co-authors include postdoctoral researchers Huimin Qiao and Yoji Nabei; Nina Balke, an associate professor of materials science and engineering; and Dali Sun, an associate professor of physics.

The material’s physical, chemical and electrical characteristics were determined using a wide-ranging combination of techniques. Computer simulations were conducted with co-authors Kinnary Patel, Sergey Prosandeev and Laurent Bellaiche at the University of Arkansas. Electron ptychography was done with co-authors Harikrishnan KP and David Muller from Cornell University. Synchrotron X-ray diffraction was done with co-authors Hua Zhou, Tao Zhou, Rui Liu and Martin Holt, from Argonne National Laboratory, with contributions from Young-Hoon Kim and Miaofang Chi at Oak Ridge National Laboratory.

The tunability of the NaNbO₃ was assessed with support from co-authors Liyan Wu, John Carroll, Cedric Meyers and Jonathan Spanier of Drexel University. Lab-source X-ray diffraction and X-ray photoelectron spectroscopy were performed with support from co-authors Aarushi Khandelwal, Kevin J. Crust, Jiayue Wang and Harold Y. Hwang of Stanford University. Second harmonic generation polarimetry measurements were conducted with co-authors Sankalpa Hazra and Venkatraman Gopalan of Penn State University.

This work was done with support from a number of sources, including the National Science Foundation under grants 2442399 and 2143642; the American Chemical Society Petroleum Research Fund under grant 68244-DNI10; the Department of Defense under grant N00014-20-1-2834; the Army Research Office, under grants W911NF-21-2-670 0162 and W911NF-21-1-0126; and the Department of Energy, under grants DE-AC02-76SF00515 and DE-SC0012375.