Piezoelectric materials are a class of smart materials that can convert electrical energy to mechanical energy and vice versa. They have been widely used in modern devices such as medical ultrasound, fuel injectors, sonar, and vibration-powered electronics. Developing new piezoelectrics for novel electromechanical device applications has been a long-lasting interest, both scientifically and technologically. Nearly all known piezoelectrics possess positive longitudinal piezoelectric coefficients: the lattice expands when applying an electric field along the polar axis. The negative response, a material contracting in response to an applied electric field, has been considered a rare and counterintuitive anomaly.  However, there is no fundamental physics preventing the realization of negative response. In the work recently published in Physical Review Letters (Editors’ Suggestion), the Geophysical Laboratory's Shi Liu and Ron Cohen showed that the negative response is not so rare after all. Detailed quantum mechanical calculations revealed that the origin of negative piezoelectric response relies on the strong ionic bonds associated with small effective charges and rigid potential energy surfaces. Furthermore, by taking advantage of the well-curated open-source materials database hosted by the Materials Project, Liu is able to quickly identify nearly 100 materials possessing this unusual piezoelectric response literally within minutes out of thousands of piezoelectric materials. It is also found that ferroelectrics with negative piezoelectric response exhibit pressure-enhanced polarization, making them appealing for high-pressure applications. This work is likely to inspire future experiential studies of this overlooked piezoelectric effect, which may offer novel avenues for designing nanoscale electromechanical devices.  

Caption: Piezoelectrics with negative longitudinal piezoelectric effect will shrink when applying an electric field (red arrow) along the direction of the polarization (yellow arrow). Though the dimension along the electric field becomes smaller, the polarization still becomes larger.