Template synthesis creates multilayer perovskites with unique ferroelectric behavior

Perovskites are a class of materials that share a specific crystal structure composed of calcium titanium oxides. Electronic devices often use perovskites because they exhibit a property called ferroelectricity. Ferroelectricity allows electric polarization to be controlled and reversed by an external electric field. This property makes perovskites useful for electronic devices such as memories, capacitors, actuators, and sensor devices that use on and off states.

To improve the functionality and reduce the environmental impact of these products, researchers are developing new compositions, structures, and lead-free ferroelectrics. Perovskites, particularly Dion-Jacobson (DJ) type layered perovskites, are becoming an important class of materials in this research.

DJ-type perovskites have a layered octahedral structure, which makes the layers asymmetric and gives them ferroelectric properties. Ferroelectric properties occur when positive and negative ions shift when an external field is applied, causing the octahedra to rotate and tilt due to size mismatches. This tilting lowers the symmetry of the material and contributes further to ferroelectric behavior.

Minoru Osada from the Institute for Sustainability Materials and Systems (IMaSS) at Nagoya University explained that researchers considered layered perovskites as unexplored materials because of the decrease in thermodynamic stability as the thickness of the perovskite layers increases.

To overcome this problem, the research group developed a new synthesis method known as the template synthesis method, which allows the synthesis of multilayered structures by layering perovskite layers one by one and aligning their octahedrons into building blocks.

“In the template synthesis method, the number of layers can be increased by one layer by using a three-layer system as the starting material and reacting with SrTiO.₃“By repeating the reaction, the number of perovskite layers can be digitally controlled according to the number of reactions, allowing the synthesis of a multilayer structure. By applying the template synthesis method, we synthesized four- and five-layer perovskites for the first time.”

Interestingly, when they tested the material, they found that it behaved strangely, exhibiting different dielectric constants and Curie temperatures depending on the number of layers.

“We found that the number of layers plays an important role in this system, and it has a unique function of switching from the traditional direct ferroelectric model when the number of layers is odd to the new indirect ferroelectric model when the number of layers is even,” Osada said.

Their approach offers a new opportunity to extend the range of ferroelectric materials beyond thermodynamically stable phases. This achievement is expected to greatly expand the field of materials exploration in the development of ferroelectrics and provide important guidelines for the development of new materials and functions that are difficult to realize with existing materials and technologies.