Intrinsic auxetic piezoelectricity in bulk ferroelectrics.

Piezoelectric materials deform under an electric field, typically exhibiting opposite signs in their longitudinal and transverse piezoelectric coefficients. Achieving the same sign for both coefficients enables the 'auxetic piezoelectric effect', where the material expands or contracts simultaneously in all directions. Although this unconventional effect has recently been observed in carefully engineered single-crystal heterostructures along specific crystallographic directions, a low-cost intrinsic bulk polycrystalline material that combines auxetic response with high-temperature stability has remained elusive. Here, we first predict and experimentally demonstrate a catalog of bulk materials exhibiting this counterintuitive intrinsic auxetic piezoelectric effect, namely the Aurivillius ferroelectrics. Theoretical analyses reveal that this effect originates from the relative sliding deformation between differently charged structural units within their layered architecture, and that it remains robust over a broad temperature range (5-1000 K). Furthermore, we show that the auxetic nature of bulk Aurivillius ceramics can be exploited to enhance the effective longitudinal piezoelectric response by an order of magnitude, achieving coexistence of strong piezoelectricity and high operating temperature. This work establishes Aurivillius ferroelectrics as a platform for auxetic piezoelectrics and opens avenues for advanced electromechanical devices.