In a new study, researchers from the Gwangju Institute of Science and Technology (GIST) in Korea investigated the transformation dynamics induced by an electric field in mixed-phase lanthanum-doped bismuth ferrite (BLFO) epitaxial thin films. They observed a connection between the presence of S/Stilt phases in BLFO film and their high piezoelectricity. These findings can help scientists design faster and more efficient piezoelectric materials.
What if electricity could be squeezed out of something? It turns out some materials have this property. Piezoelectricity is the electric charge that accumulates in certain solids when mechanical stress is applied on them. Piezoelectric materials, like bismuth ferrite thin films, when grown on a single lanthanum aluminate substrate give rise to highly strained epitaxial thin films that exhibit excellent electromechanical and ferroelectric properties. In bismuth ferrite thin films “doped” or polluted with lanthanum (BLFOs), piezoelectricity is attributed to the presence of “mixed-phase structures” with stripe patterns.
The formation of stripe patterns and controlling the mixed-phase structures of BLFO have been the focus of many studies over the years. But due to the ultrafast nature of phase transitions, the formation of energetically “favorable” phases under applied electric field and the origin of large electrochemical response has not been sufficiently explored. Many scientists engaged in research on BLFO are currently plagued by the question, what does the presence of an S-polymorph, an intermediate phase, do to the properties of the material?
Researchers at GIST, led by Professor Ji Young Jo, began investigating the phase transformation dynamics of BLFO epitaxial thin films with the help of time-resolved X-ray microdiffraction. “We chose this technique because it helps us understand the electric field-induced phase transformation dynamics of the piezoelectric materials in a time scale ranging from picoseconds to microseconds,” said Professor Jo. The results of their exploration of the piezoelectric properties of BLFO films along with the identification of the mixed-phase structures and striped patterns were published in Acta Materialia on April 1, 2021, and were made available online on Jan 21, 2021.
BLFO can be converted into monoclinic (MA, MC, tilted MC), tetragonal (T-phase), an intermediate S-phase or mixed phases via strain engineering. The investigation into the transformation dynamics revealed that the phase change from Mc to S-phase were dependent on the polarity of the electric field applied. The study also concluded that the high piezoelectric response seen in mixed-phase BLFO films is due to the presence of S/Stilt phases.
“Understanding the role of stripe patterns and the S-phases can help us create ultrafast piezoelectric devices with a response time of sub-microseconds,” concludes Professor Jo. The findings from this study provide a new perspective on the use of strain engineering to design ultra-high piezoelectric thin films. This has far-reaching implications for the future of energy harvesting.
Authors: Jun Young Lee, Gopinathan Anoop, Sanjith Unithrattil, WooJun Seol and Ji Young Jo, all of School of Materials Science and Engineering, GIST; Youngki Yeo and Chan-Ho Yang, of the Department of Physics & Center for Lattice Defectronics, KAIST, Daejeon 34141; and Su Yong Lee of the Pohang Accelerator Laboratory, Pohang 37673, South Korea
Ji Young Jo is a professor of Materials Science and Engineering at the Gwangju Institute of Science and Technology. Her group investigates the electric field-induced dynamics of ferroelectric epitaxial thin films. In 2008, she received a Ph.D. in physics from the Seoul National University. Before joining GIST, she completed postdoctoral training at the University of Wisconsin-Madison.