The relaxor-ferroelectric material is a high-performance ultrasound generating element that exhibits a large dielectric response due to its complex structure. Recently, researchers from Japan used a resolved Raman microscope with a novel polarization angle developed by them to investigate the distribution of electrical polarization in lead-magnesium niobate crystals of lead titanate, a piezoelectric material used in ultrasound equipment and fish-finding probes. Information derived from electrical polarization has the potential to enhance the performance of the next generation of ultrasound diagnostic devices.
Exploitation of polarization or charge separation in ferroelectric materials has resulted in remarkable advances in various fields, such as the development of new ultrasound diagnostic devices. Most notably, these ferroelectric materials have resulted in piezoelectric devices capable of converting electrical signals into mechanical motion. Understanding how electrical polarization is regulated and fluctuates within a material is key to building better devices. However, disturbances in the arrangement of atoms together with their inhomogeneous structures can lead to disordered charge distribution in certain regions, which is a fundamental challenge for the development of ferroelectric materials.
To visualize the effect of interference on polarization behavior, researchers led by Professor Yasuhiro Fujii of Ritsumeikan University, Japan, have developed an innovative polarization angle-resolved Raman microscope. This patented technique builds on the principle of Raman microscopy and involves directing a focused laser beam onto a sample and analyzing the scattered light to understand the molecular structure of the material. Unlike traditional microscopy, the new technique incorporates a rotating half-wave plate into the microscope setup to consider the polarizing effect of light without rotating the sample under study. This new approach produces a spectrum with a different direction of light polarization at each point in the sample under study. Combining the spectral data makes it possible to identify not only the vibrational states of atoms but also the direction of vibrations in materials.
So, in research published in the journal Communications Physics on May 18 2023 led by Prof. Shinya Tsukada from Shimane University and Prof. Fujii, the researchers used this technique to observe the electrical polarization array and its fluctuation time scale in the piezoelectric magnesium niobate lead (Pb(Mg1/3Nb2/3)HI3)-lead titanate (PbTiO3) or PMN-PT crystals, used in diagnostic ultrasound equipment, reveal the reason for the large dielectric constant.
“The development of a resolved Raman microscope with this polarization angle together with advances in analytical techniques could enable the incorporation of polarization information into existing Raman imaging data and enable a deeper understanding of material properties,” explained Prof. Fujii, talking about the reasons behind the development of this technique.
One of the important characteristics of PMN-PT crystals is the apparent dielectric and piezoelectric response at the boundaries separating the different phases in the material. The specific composition of PMN-PT crystals, especially the concentration of titanium (Ti), can influence the formation and phase boundary characteristics. To investigate the effect of the mixing ratio of Ti 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 microscopy.
The Ti content varies from 27.0 mol% to 38.0 mol% throughout the sample, giving rise to three distinct phases: the monoclinic (type B) phase in which the Ti content ranges between 27.0 mol and 29.2 mol%, monoclinic (type C). ) where it reaches 34.5 mol%, and the tetragonal phase with a high Ti content of 34.8–38.0 mol%.
When analyzing the Raman spectrum corresponding to the different polarization values of light at each point in the sample, the researchers observed an abrupt change in the intensity of the Raman peak only for the monoclinic B-type phase. Moreover, they also noted the distinct changes in the direction of spontaneous polarization in this phase. The spectrum reveals a slower relaxation (reorientation of the electric dipole in response to thermal perturbation) of the polarization of the material closer to the phase boundary between the monoclinic (type B) and (type C) phases. This, in turn, suggests that dipole realignment occurs at a reduced rate, enabling the material to store large amounts of charge and display an enhanced dielectric response at this phase boundary.
“We found that the ability of the relaxor-ferroelectric material to store large amounts of electric charge is due to the slow response of nanometer-scale electrical polarization to external voltages,” highlighted Prof. Fujii.
In summary, observations of these characteristic properties of relaxor materials highlight the ability of polarization angle resolving microscopy to provide polarization information, which can help optimize the material’s dielectric performance. In particular, insight into the polarization behavior of PMT-PT could enhance the development of relaxor materials with better ultrasound detection and excitation properties for next-generation diagnostics.