Developing novel perovskite-type ferroelectric materials

Researchers from Tohoku University have engineered a novel displacement-type ferroelectric material with notable dielectric properties. Their research findings include the successful synthesis of rubidium niobate (RbNbO₃), a compound previously deemed challenging to produce under pressures exceeding 40,000 atmospheres. The researchers also characterised how polarisation changes across a wide temperature range during phase transitions. The research findings could lead to new design guidelines for ferroelectric materials.

Capacitors are crucial components in a range of electronic devices. They are made of dielectric materials that polarise on the application of the voltage. Currently, barium titanate (BaTiO₃) is reportedly 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 behaviour, 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, researchers led by Ayako Yamamoto from the Shibaura Institute of Technology have developed a displacement-type ferroelectric material with a high dielectric constant.

Employing a high-pressure method, the researchers incorporated sizeable rubidium ions into perovskite-type compounds, resulting in the synthesis of rubidium niobate. This compound, previously known for its challenging synthesis process, was created through an innovative approach. RbNbO₃ exhibits displacement ferroelectricity like BaTiO₃, making it a suitable candidate for capacitors. 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, 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,” Yamamoto said.

The researchers synthesised non-perovskite-type RbNbO₃ by sintering a mixture of rubidium carbonate and niobium oxide at 800°C, then subjected it to high pressures of 40,000 atmospheres at 900°C for 30 minutes. Under these 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.

The researchers then used X-ray diffraction to investigate the crystal structure, revealing that the 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 polarisation due to phase transitions.

Through powder X-ray diffraction, the researchers identified four distinct phase transitions occurring across temperatures ranging from -268 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 200°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.

These phase transitions closely match the predictions made through first-principles calculations. Yamamoto said the high-pressure phase obtained by the researchers confirmed the presence of a polar structure from the observation of second harmonic generation of the same strength as potassium niobate, while 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,” Yamamoto said.

The advantage of the high-pressure method lies in its ability to stabilise 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.

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