A team of researchers from from Penn State, the University of California, Oak Ridge National Laboratory and Rutgers University have observed, for what they say is the first time, the unique microstructure of a novel ferroelectric material, enabling the development of lead-free ferroelectric materials for electronics, sensors, and energy storage that are safer for human use.
Ferroelectrics are a class of materials that demonstrate a spontaneous electric polarization as the result of shifts of negative and positive charges that can be reoriented within the material with application of external force. They are especially useful for data storage and memory as they can remain in one polarized state without additional power, making them attractive for energy-saving data storage and electronics.
The research team worked with calcium manganese, Ca3Mn2O7 (CMO), a novel hybrid improper ferroelectric material with some interesting properties. CMO is a Ruddlesden-Popper phase layered perovskite structure. Perovskites crystals have a unique structure that makes them attractive for ferroelectrics, and Ruddlesden-Popper phase perovskites feature two layers of perovskite that are interleaved with positive-charge ions. In these materials, atoms of oxygen form octahedrals that can tilt and rotate.
"The designing principle of this material is combining the little octahedrals,” said Liexin Miao, doctoral candidate in materials science and co-author of the study in Nature Communications. “In the material, there are little octahedrals of oxygen atoms that can tilt and rotate. The term ‘hybrid improper ferroelectric’ means we combine the rotation and the tilting to produce ferroelectricity, so it is considered hybrid and because the polarization is generated as a secondary effect, it is considered an ‘improper’ ferroelectric. It is the two motions of the octahedron generating that polarization for ferroelectricity.”
There are some unique characteristics that make this material of special interest to researchers. “At room temperature, there are some polar and nonpolar phases coexisting at room temperature in the crystal,” Miao said. “And those coexisting phases are believed to be correlated with negative thermal expansion behavior. It is well-known that normally, a material expands when heated, but this one shrinks. That is interesting, but we know very little about the structure, like how the polar and nonpolar stuff coexists.”
To explore this further, the researchers used atomic-scale electron microscopy. "Why we used electron microscopy is because with electron microscopy, we can use atomic-scale probes to see the exact atomic arrangement in the structure,” Miao said. “And it was very surprising to observe the double bilayer polar nanoregions in the CMO crystals. To our knowledge, it is the first time that such microstructure was directly imaged in the layered perovskite materials.”
The team explained that what happens to a material that goes through such a ferroelectric phase transition was never observed before. But with electron microscopy, they could monitor the material and what was happening during the phase transition.
"We monitored the material, what’s going on during the phase transition, and were able to probe atom by atom at what type of bonding we have, what type of structural distortions we have in the material, and how that may change as a function of temperature,” said Nasim Alem, Penn State associate professor of materials science and engineering and the study’s corresponding author. “And this is very much explaining some of the observations that people have had with this material. For example, when they get the thermal expansion coefficient, no one has really known where this comes from. What is the design physics? What is the atomic structure? What type of atomic structure and bonding do we have that leads to this type of behavior?"
“Basically, this was going down into the atomic level and understanding the underlying atomic scale physics, chemistry and also the phase transition's dynamics, how it's changing.”
“First of all, we used one of the most powerful aberration-corrected TEMs, the ThermoFisher Titan G2, for the atomic resolution imaging and spectroscopy,” Miao said. “Due to the nanometer scale size of the polar nanoregions, it would not be possible for us to uncover such unique structure without the atomic resolution TEM images. Second, we used the in-situ TEM capability at MRI’s Materials Characterization Lab (MCL) for our in-situ experiments, which allowed us to explore the dynamics of the phase competitions in the heating environment. And third, we have amazing, focused ion beam equipment in the MCL that allows us to make beautiful TEM specimens with a thickness as low as around 20 nanometers.”
The potential benefits of this research include the possibility of lead-free piezoelectric crystals. Currently, piezoelectric crystals contain lead, which is toxic for humans and animals.
“Piezoelectric materials are widely used in applications such as switches, computer memory, energy storage, actuators and we would love to design a material that doesn’t have the disadvantages of the current materials,” Alem said. “And right now, lead in all these materials is a big disadvantage because the lead is hazardous. We hope that our study can result in a suitable candidate for a better piezoelectric system.”
A benefit for fellow researchers that came out of the study is free software developed by the research team, EASY-STEM, that enables easier TEM image data processing.
“The software has a graphical user interface that allows users to input with mouse clicks, so people do not need to be an expert in coding but still can generate amazing analysis,” Miao said.