The ability of piezoelectric materials to reversibly convert electric to mechanical energies makes them ubiquitous in daily life usages, such as the quartz crystal in quartz watches, and also critical for advanced applications, including medical ultrasound and underwater acoustics.

It is also desirable to fabricate transparent piezoelectric materials for hybrid applications such as photoacoustic imaging. However, the most commonly used transparent piezoelectric material (LiNbO₃) has only moderate piezoelectric activity (~40 pico-coulomb per newton) while the best piezoelectric crystal (lead magnesium niobate-lead titanate, or PMN-PT) is usually opaque or translucent, at best. It seems that one has to make a compromise between ultrahigh piezoelectricity and excellent optical clarity.

This dilemma is worked out in our recent study. We demonstrate that both ultrahigh piezoelectricity (> 2100 pico-coulomb per newton) and near-perfect transparency can be simultaneously realized in high-quality PMN-PT single crystals by putting the crystal under an alternating current (a.c.) electric field.

We conceived this idea by performing computer modeling and simulations using the phase-field method, which is implemented in commercialized software µ-pro^® from Penn State (http://mupro.co/contact/). The initial state of the modeled PMN-PT crystal is featured by myriads of microscopic regions consisting of nicely aligned electric dipoles, known as the domains, and high-density boundaries between the domains, known as the domain walls. We found that, after the application of a few cycles of a.c. electric fields, the domain size becomes bigger and bigger with the elimination of certain domain walls while the piezoelectric activity considerably increases. Noticeably, our finding that better piezoelectricity can be achieved by making domains larger is contrary to the common belief in the community of piezoelectric research.

Fig. Schematics of the light transmittance through PMN-PT single crystals poled by d.c. and a.c. electric fields. The domain structures are obtained from phase-field simulations. The figure is adapted from the News and Views “Transparent crystals with ultrahigh piezoelectricity” (https://www.nature.com/articles/d41586-020-00038-z). Credit: Xiaoxing Cheng/Penn State.

Our theoretical predictions are consolidated experimentally: an up-to 30% enhancement of the piezoelectricity of PMN-PT crystals is obtained after the treatment of an a.c. electric field. Moreover, optical characterizations by using High-Resolution X-ray Diffraction, Birefringence Imaging Microscopy, and Polarized Light Microscopy techniques also provide ample evidence for the enlargement of domains.

The increased domain size during a.c. poling also leads to an unexpected outcome – excellent light transparency of the crystal due to the elimination of the light-scattering domain walls. Moreover, the a.c.-poled crystal also exhibits other ideal physical properties including excellent electromechanical coupling factor (k₃₃ ~ 94%), large electro-optical coefficient (γ₃₃ ~ 220 picometers per volt), and good temperature stability up to 95 °C.

The simultaneous realization of ultrahigh piezoelectricity and near-perfect optical transparency is unprecedented and has many promising applications including haptic devices, medical imaging and diagnosis, and invisible robotics. The alternating polarized layered structure of these crystals also possesses potentials for optical applications such as tunable terahertz-wave generation.