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Unexpected soft ferroelectric domain walls

"Our colleagues at ICN2 studied a range of ferroelectrics using atomic force microscopy (AFM) and discovered that 180-degree domain walls – regions separating ferroelectric domains with opposite polarization – are mechanically softer than the surrounding material" says Konstantin Shapovalov, ICMAB researcher and co-author of the now published article in Physical Review X Mechanical Softness of Ferroelectric 180° Domain Walls, in which he contributed with the theoretical interpretation of this unexpected phenomenon. 

29 October 2020

"Any ferroelectric material has several polarization states that can coexist with each other, often forming intricate domain structures. Lately there has been a lot of research focused on domain walls – the 2D entities separating domains with different polarization. Because domain walls have a different structure from the surrounding material, they can introduce new functionalities: some walls can be 2D conducting pathways in an otherwise insulating material, others are known to improve the piezoelectric response of the whole material – and the list goes on.

What has been overlooked up to this point is the mechanical properties of domain walls. Our colleagues at ICN2 studied the so-called 180-degree domain walls (walls that separate domains with opposite polarization) using AFM in several ferroelectric materials and showed that they are mechanically softer than the surrounding material. This is surprising at first because the domains separated by such walls are completely identical mechanically and one would not normally expect the domain walls to behave differently.

Me (Konstantin Shapovalov) and Max Stengel developed a theory that helped to understand why such domain walls are softer. By doing a series of finite-element simulations we showed that piezoelectricity plays a large part in this phenomenon. Whenever an inhomogeneous strain is introduced to a ferroelectric (like when an AFM tip is pressed), some of the mechanical energy is converted into electrostatic energy via the piezoelectric effect, which makes the material stiffer. We showed that domain walls and their vicinity effectively have a lower piezoelectric constant than the surrounding material and therefore are naturally expected to be mechanically softer.

In the end of the day, we demonstrated a way to detect domain walls just by using AFM, without necessity of applying electric fields, and we showed that a range of domain walls are in fact mechanically distinct entities promising for new functionalities (like, hypothetically, phonon waveguides)."

Text written by Konstantin Shapovalov, ICMAB researcher at the Laboratory of Electronic Sturcture of Materials Group

The study published in Physical Review X was performed by researchers of the Catalan Institute of Nanoscience and Nanotechnology (ICN2) in collaboration with researchers of the Institute of Materials Science of Barcelona (ICMAB), the Italian Institute of Technology, the Scuola Normale Superiore di Pisa (Italy) and  Cornell University (USA). 

For more information, read the news post at the ICN2 website: "New studies on ferroelectric domain walls reveal peculiar mechanical properties" and the article referenced below. 

Reference article:

Mechanical Softness of Ferroelectric 180° Domain Walls
Christina Stefani, Louis Ponet, Konstantin Shapovalov, Peng Chen, Eric Langenberg, Darrell G. Schlom, Sergey Artyukhin, Massimiliano Stengel, Neus Domingo, and Gustau Catalan
Phys. Rev. X 10, 041001, October 2020.
DOI: 10.1103/PhysRevX.10.041001


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