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Towards Novel Promising Perovskite-Type Ferroelectric Materials

Researchers have pioneered a breakthrough in ferroelectric material development. They've engineered a novel displacement-type ferroelectric material boasting remarkable dielectric properties. Their achievement includes the successful synthesis of rubidium niobate (RbNbO3), a compound previously deemed challenging to produce under pressures exceeding 40,000 atmospheres. Additionally, they characterized how polarization changes across a wide temperature range during phase transitions. This breakthrough can lead to new design guidelines for ferroelectric materials.

Capacitors are crucial components in electronic devices such as smartphones and computers. They are made of dielectric materials that polarize on the application of the voltage. Currently, barium titanate (BaTiO3) is the most widely used material for capacitors. Barium titanate belongs to the perovskite group of materials, where a titanium ion resides within an oxygen octahedral cage. The material exhibits displacive-type ferroelectric behavior, where the displacement of ions during the phase transition leads to the creation of a permanent dipole moment within the material.

In a study published in the journal Dalton Transactions on April 1, 2024, researchers led by Ayako Yamamoto from the Shibaura Institute of Technology, and including graduate student Kimitoshi Murase, have developed a displacement-type ferroelectric material with a high dielectric constant. The theoretical part was investigated by Hiroki Moriwake and his group from the Japan Fine Ceramics Center.

Employing a high-pressure method, researchers successfully incorporated sizable rubidium ions into perovskite-type compounds, resulting in the synthesis of rubidium niobate (RbNbO3). This compound, previously known for its challenging synthesis process, was effectively created through an innovative approach. RbNbO3 exhibits displacement ferroelectricity like BaTiO3, making it a promising candidate for capacitors.s. However, investigations into its dielectric properties have only been conducted at low temperatures (below 27°C). This study sheds light on the crystal structure and phase transitions across a broad temperature range (-268 to +800°C), paving the way for further research and development.

"The high-pressure synthesis method has reported a variety of materials with perovskite-type structures, including superconductors and magnets. In this study, our focus was on combining niobates and alkali metals known for their high dielectric properties," says. Yamamoto.

The researchers synthesized non-perovskite-type RbNbO3 by sintering a mixture of rubidium carbonate and niobium oxide at 1073 K (800°C), then subjected it to high pressures of 40,000 atmospheres at 1173 K (900°C)for 30 minutes. Under these high-pressure and high-temperature conditions, the rubidium niobate underwent a structural transformation from a complex triclinic phase at ambient pressure phase into a 26 % denser orthorhombic perovskite-type structure.

Using X-ray diffraction, the researchers investigated the crystal structure. Their analysis using a single crystal revealed that the crystal structure closely resembled that of potassium niobate (KNbO3) and exhibited similar distortions observed in BaTiO3, both well-known ferroelectric materials. However, they found that the orthorhombicity and displacement of niobium atoms in RbNbO3 exceeded those of KNbO3, indicating a higher degree of dielectric polarization due to phase transitions.

Furthermore, through powder X-ray diffraction, researchers identified four distinct phase transitions occurring across temperatures ranging from -268°C to +800°C. Below room temperature, RbNbO3 exists in an orthorhombic phase, which is the most stable configuration. As the temperature rises, it undergoes transitions: first to a tetragonal perovskite phase above 220°C, then into a more elongated tetragonal perovskite phase beyond 300°C. Finally, above 420°C, it reverts to a non-perovskite phase found under atmospheric conditions.

The researchers investigated crystal structure and properties of perovskite-RbNbO3 prepared at 4 GPa. Single-crystal XRD analysis revealed RbNbO3 exhibits the same non-centrosymmetric space group as ferroelectrics BaTiO3 and KNbO3. ©Ayako Yamamoto.

These observed phase transitions closely match predictions made through first-principles calculations. The researchers also calculated the dielectric polarization of different phases of RbNbO3. They found that the orthorhombic phase had a polarization of 0.33 C m-2, while the two tetragonal phases showed polarizations of 0.4 and 0.6 C m-2, respectively. These values are comparable to those of ferroelectric alkali metal niobates such as KNbO3 (0.32 C m-2), LiNbO3 (0.71 C m-2), and LiTaO3 (0.50 C m-2).

"The high-pressure phase obtained this time confirmed the presence of a polar structure from the observation of second harmonic generation of the same strength as potassium niobate, and a relatively high relative permittivity was also obtained. As for the dielectric constant, it is expected that values equal to or greater than those of potassium niobate can be obtained by increasing the sample density, as predicted from theoretical calculations," says Professor Yamamoto.

The researchers are planning further experiments to accurately measure the dielectric constant and demonstrate the high polarization of RbNbO3. The advantage of the high-pressure method lies in its ability to stabilize substances that do not exist under atmospheric pressure. Using the proposed method, larger alkali metal ions such as cesium could be incorporated into the perovskite structure, leading to ferroelectrics with desirable dielectric properties.

This study was supported by the Kazuchika Okura Memorial Foundation's research grant in 2022-2023 and Innovative Science and Technology Initiative for Security grant number JPJ004596, ATLA, Japan. This study was also supported by the GIMRT program (202212-RDKGE-0018) at the Institute for Materials Research, Tohoku University, and a Joint Research Program (202306-MCBXG-0079) at the Institute of Solid State Physics, University of Tokyo. Powder XRD measurements were performed at Techno-plaza at Shibaura Institute of Technology.

Publication Details:

Title: Crystal structure and properties of perovskite-type rubidium niobate, a high-pressure phase of RbNbO3
Authors: Ayako Yamamoto, Kimitoshi Murase, Takeru Sato, Kazumasa Sugiyama, Toru Kawamata, Yoshiyuki Inaguma, Jun-ichi Yamaura, Kazuki Shitara, Rie Yokoie and Hiroki Moriwake
Journal: Dalton Transactions
DOI: 10.1039/d4dt00190g


Saeko Komatsu

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