Researchers from the Shibaura Institute of Technology 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 difficult to supply under pressures exceeding 40,000 atmospheres. Moreover, they characterised how polarization changes across a large temperature range during phase transitions. This breakthrough can result in latest design guidelines for ferroelectric materials.
Capacitors are crucial components in electronic devices similar to smartphones and computers. They’re manufactured from dielectric materials that polarize on the appliance of the voltage. Currently, barium titanate ((BaTiO₃) is essentially the most widely used material for capacitors. Barium titanate belongs to the perovskite group of materials, where a titanium ion resides inside an oxygen octahedral cage. The fabric exhibits displacive-type ferroelectric behavior, where the displacement of ions through the phase transition results in the creation of a everlasting dipole moment inside the material.
In a study published within the journal Dalton Transactions, 2024, 53, 7044 -7052 on April 1, 2024, researchers led by Professor Ayako Yamamoto from the Shibaura Institute of Technology, including master student Kimitoshi Murase have developed a displacement-type ferroelectric material with a high dielectric constant. The theoretical part was investigated by Dr. Hiroki Moriwake and his group from the Japan Tremendous Ceramics Center.
Employing a high-pressure method, researchers successfully incorporated sizable rubidium ions into perovskite-type compounds, leading to the synthesis of rubidium niobate (RbNbO3). This compound, previously known for its difficult synthesis process, was effectively created through an revolutionary approach. RbNbO3 exhibits displacement ferroelectricity like BaTiO3, making it a promising candidate for capacitors and interest in synthesizing RbNbO3 dates back to the Nineteen Seventies. Nonetheless, 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 best way for further research and development.
“The high-pressure synthesis method has reported quite a lot of materials with perovskite-type structures, including superconductors and magnets. On this study, our focus was on combining niobates and alkali metals known for his or her high dielectric properties,” says Prof. Yamamoto.
The researchers synthesized non-perovskite-type RbNbO3 by sintering a combination 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 half-hour. Under these high-pressure and high-temperature conditions, the rubidium niobate underwent a structural transformation from a posh triclinic phase at ambient pressure phase right into a 26 % denser orthorhombic perovskite-type structure.
Using X-ray diffraction, the researchers investigated the crystal structure. Their evaluation using a single crystal revealed that the crystal structure closely resembled that of potassium niobate (KNbO3) and exhibited similar distortions observed in BaTiO3, each well-known ferroelectric materials. Nonetheless, they found that the orthorhombicity and displacement of niobium atoms in RbNbO3 exceeded those of KNbO3, indicating a better degree of dielectric polarization resulting from phase transitions.
Moreover, through powder X-ray diffraction, researchers identified 4 distinct phase transitions occurring across temperatures starting from -268°C to +800°C. Below room temperature, RbNbO3 exists in an orthorhombic phase, which is essentially the most stable configuration. Because the temperature rises, it undergoes transitions: first to a tetragonal perovskite phase above 220°C, then right 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.
These observed phase transitions closely match predictions made through first-principles calculations. The researchers also calculated the dielectric polarization of various phases of RbNbO3. They found that the orthorhombic phase had a polarization of 0.33 C m−2, while the 2 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 similar to 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 remark of second harmonic generation of the identical strength as potassium niobate, and a comparatively high relative permittivity was also obtained. As for the dielectric constant, it is predicted that values equal to or greater than those of potassium niobate will be obtained by increasing the sample density, as predicted from theoretical calculations,” says Prof. Yamamoto.
The researchers are planning further experiments to accurately measure the dielectric constant and reveal the high polarization of RbNbO3. The advantage of the high-pressure method lies in its ability to stabilize substances that don’t exist under atmospheric pressure. Using the proposed method, larger alkali metal ions similar to cesium may very well be incorporated into the perovskite structure, resulting in ferroelectrics with desirable dielectric properties.
