Vibrational spectroscopy of lead-free potassium sodium niobate and related perovskite ferroelectrics

The potassium sodium niobate (K_xNa_1−x)NbO₃ (KNN) family with the perovskite structure is a technologically important lead-free piezoelectric material. At first, the nature of end members, the physical properties, and phase transitions of simple alkali niobate materials MNbO₃ (M=Li, Na, K, Rb, and Cs) are reviewed. Secondly the binary solid solution's phase diagram, KNN, is discussed concerning the morphotropic phase boundary (MPB) near x=0.5. To understand the phase transitions near the MPB composition, the temperature dependences of lattice dynamical properties of KNN single crystals on optical modes and acoustic modes are reported using Raman and Brillouin scattering spectroscopies, respectively. Physical properties and phase transitions of KNN-based solid solutions were also reviewed. These new features may give new insights into the development of lead-free piezoelectric and ferroelectric materials.

Vibrational spectroscopy is a powerful tool to investigate the dynamical atomic motion of condensed matter. Raman scattering is the inelastic light scattering by optical phonons. It is a powerful tool to investigate crystal symmetry and structural changes, because optical mode frequencies are very sensitive to crystal structures (https://doi.org/10.3390/solids5040040). Brillouin scattering is another inelastic light scattering by acoustic phonons. Acoustic phonons are closely related to elastic properties, which reflect crystal structure and bonding. Such elastic properties are very important in understanding the lattice stability of piezoelectric and ferroelectric materials. Therefore, the temperature dependence of the elastic properties of  ferroelectric phase transitions has been investigated by Brillouin scattering (https://doi.org/10.3390/ma15103518). Professor Seiji Kojima from the University of Tsukuba has studied various ferroelectric and piezoelectric materials using Raman and Brillouin scattering spectroscopies.

Ferroelectricity is defined by the existence of a spontaneous polarization and the change of the direction of polarization by an external electric field. Ferroelectricity is related to a ferroelectric phase transition from a high-temperature paraelectric phase with no spontaneous polarization into a ferroelectric phase by cooling. Ferroelectric materials have many attractive functionalities, such as pyroelectric, piezoelectric, nonlinear optic, and elasto-optic effects. They have been extensively applied to various kinds of devices, such as ultrasonic transducers, sensors, filters, memories, and condensers. Various perovskite oxide ferroelectrics have been developed due to their excellent functionality and chemical stability, and are very important in the industry. Since the technologically important piezoelectric and ferroelectric materials include toxic lead, such as Pb(Zr_xTi_1-x)O₃ (PZT), the development of lead-free materials has attracted much attention.

It is important to replace Pb at the A-site for the development of lead-free perovskite ferroelectrics. For the first step, the phase transitions and physical properties of lead-free simple alkali niobate materials MNbO₃ (M=Li, Na, K, Rb, and Cs) are reviewed on experimental and theoretical studies based on the tolerance factor of a perovskite structure, which is sensitive to the ionic radius of alkali ions.

Next, the phase diagram of binary solution (K_xNa_1-x)NbO₃ (KNN) and morphotropic phase boundary (MPB) at x=0.5 is discussed. The successive phase transition of KNN single crystals with x=0.5 is discussed using Raman and Brillouin scattering spectroscopies. The temperature dependence of low-frequency optical phonon modes shows the lattice instability. The changes of crystal symmetry are related to the high-frequency internal modes. The elastic properties and elastic anomalies of KNN single crystals in the vicinity of structural phase transitions are discussed by Brillouin scattering spectroscopy.

Finally, Raman scattering has also been used to study the physical properties and phase boundaries of KNN-based solid solutions such as (1 − x − y)(K0.5Na0.5)NbO-xBZrO3-yATiO3. Ferroelectric and structural phase transitions of their binary KNN solid solutions are described with the change of crystal symmetry and the morphotropic phase boundary (MPB). 

These new physical properties of the KNN family give new insights into the further development of lead-free piezoelectric and ferroelectric materials concerning applications in electrical engineering, electronics, and signal processing.

The paper was published in Electron. Signal Process.

Kojima S. Vibrational spectroscopy of lead-free potassium sodium niobate and related perovskite ferroelectrics. Electron. Signal Process. 2025(1):0002