A newly developed polarization-angle-resolved Raman microscope could help enhance the performance of relaxor–ferroelectric materials for next-generation diagnostics

Relaxor–ferroelectric materials are high-performance ultrasound generation elements exhibiting large dielectric responses owing to their complex structures. Recently, researchers from Japan made use of a novel polarization-angle-resolved Raman microscope developed by them to investigate the electric polarization distribution in lead magnesium niobate-lead titanate crystals, a piezoelectric material used in ultrasound equipment and fish finder probes. The information obtained on electric polarization holds the potential for performance enhancement of next-generation ultrasound diagnostic devices.

The exploitation of polarization or charge separation in ferroelectric materials has led to remarkable advances in various fields, such as the development of new ultrasound diagnostic devices. Prominently, these ferroelectric materials have led to piezoelectric devices capable of transforming electric signals into mechanical motion. Understanding how the electric polarization is arranged and fluctuates in a material is key to building better devices. However, disorders in the atomic arrangement along with their inhomogeneous structures can lead to irregular charge distribution in specific regions, posing a fundamental challenge to the development of ferroelectric materials.

To visualize the effect of disorders on polarization behavior, researchers led by Professor Yasuhiro Fujii from Ritsumeikan University, Japan, have developed an innovative polarization-angle-resolved Raman microscope. This patented technique builds upon the principles of Raman microscopy and involves directing a focused laser beam onto a sample and analyzing the scattered light to understand the molecular structure of materials. Unlike traditional microscopes, the new technique incorporates a rotating half-wave plate into the microscope setup to consider the effects of light polarization without the need to rotate the sample under study. This novel approach produces spectra with different light polarization directions at each point in the sample under study. Combining the spectral data makes it possible to identify not only the vibration states of atoms but also the vibration directions in the material.

Now, in a study published in the journal Communications Physics on 18 May 2023 led by Prof. Shinya Tsukada from Shimane University and Prof. Fujii, researchers have used this technique to observe the arrangement of the electric polarization and the time scale of its fluctuation in a piezoelectric lead magnesium niobate [Pb(Mg_1/3Nb_2/3)O₃]-lead titanate [PbTiO₃] or PMN-PT crystal, which is used in diagnostic ultrasound equipment, revealing the reason for the large dielectric constant.

“The development of this polarization-angle-resolved Raman microscope along with the advancements in analytical techniques can enable the incorporation of polarization information into the existing Raman imaging data and allow a deeper understanding of material properties,” explains Prof. Fujii, speaking of the rationale behind the development of this technique.

One notable characteristic of PMN-PT crystals is their pronounced dielectric and piezoelectric response at the boundaries that separate different phases in the material. The specific composition of the PMN-PT crystal, particularly the concentration of titanium (Ti), can affect the formation and characteristics of phase boundaries. To investigate the effect of Ti mixing ratios on the dielectric properties, the researchers imaged a 62.7 × 15.0 × 0.3 PMT-PT crystal sample with the newly developed setup for Raman mapping in the microscope.

The Ti content varied from 27.0 mol% to 38.0 mol% along the length of the sample, giving rise to three distinct phases: a monoclinic (type B) phase where the Ti content ranged from 27 mol% and 29.2 mol%, a monoclinic (type C) phase where it went up to 34.5 mol%, and a tetragonal phase with a high Ti content of 34.8–38.0 mol%.

On analyzing the Raman spectra corresponding to different light polarization values at each point in the sample, the researchers observed abrupt changes in the intensity of the Raman peaks only for the monoclinic type B phase. Moreover, they also noted a distinct change in the direction of spontaneous polarization in this phase. The spectra revealed a slower relaxation (reorientation of the electric dipoles in response to a thermal perturbation) of the material’s polarization closer to the phase boundary between the monoclinic (type B) and (type C) phases. This, in turn, indicated that the realignment of the dipoles occurs at a reduced rate, enabling the material to store a large amount of charge and display enhanced dielectric response at this phase boundary.

“We found that the ability of the relaxor–ferroelectric material to store a significant amount of electric charge is due to the slow response of nanometer-scale electric polarization to the external voltage,” highlights Prof. Fujii.

In summary, the observation of this characteristic property of relaxor materials highlights the capability of the polarization-angle-resolved microscope to provide polarization information, which could help optimize a material’s dielectric performance. In particular, the insights into the polarization behavior of PMT-PT could enhance the development of relaxor materials with improved ultrasound detection and generation properties for next-generation diagnostics.

Title of original paper: Polarization behavior in a compositionally graded relaxor–ferroelectric crystal visualized by angle-resolved polarized Raman mapping
Journal: Communications Physics
DOI: 10.1038/s42005-023-01219-8

About Professor Yasuhiro Fujii from Ritsumeikan University, Japan

Yasuhiro Fujii is a lecturer at the Department of Physical Sciences, Faculty of Science and Engineering at Ritsumeikan University since 2019. He received his Ph.D. Degree in Science from Osaka University in 2006. He specializes in inelastic light scattering spectroscopy, and his recent research interests are ferroelectric phase transitions, interaction dynamics of hydrated water molecules, and intermediate-range order in glasses. He has co-authored two books on optical techniques, including Raman spectroscopy. In addition, he has published twenty-four papers between 2019 and 2022. He was awarded the Young Scientist Award of the Physical Society of Japan for his contribution to developing angle-resolved Raman spectroscopy in 2018.

About Associate Professor Shinya Tsukada from Shimane University, Japan

Shinya Tsukada is an Associate Professor at the Faculty of Education and Graduate School of Natural Science and Technology, Shimane University since 2017. Dr. Tsukada obtained his PhD in Engineering from the University of Tsukuba, Japan, in 2009. His research interests encompass statistical physics and materials science, with a particular focus on ferroelectric phase transitions, the observation of order parameter fluctuations through light scattering, and dielectric and other measurements. To date, he has published more than 85 papers on various topics such as ferroelectricity, spectroscopy, crystal growth, and ceramics fabrication. He was conferred the Young Scientist Award by Shimane University in 2018.

Funding information

This study was partly supported by JSPS KAKENHI Grant Nos. 19H02618, 19K05252, 21H01018, and 22H01976, and Shimane University Internal Competitive Grants of FY2021. Sample evaluations using synchrotron radiation were performed at the BL22XU beamline of the SPring-8 facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals No. 2012A3713 and No. 2013A3713).