Origin of Negative Longitudinal Piezoelectric Effect

Piezoelectrics are a class of high-coupling functional materials that can convert mechanical energy to electrical energy and vice versa. The longitudinal piezoelectric coefficients along the polar axis are almost always positive. Using density functional theory calculations, we demonstrate that several hexagonal ABC ferroelectrics possess significant negative longitudinal piezoelectric effect: the lattice contracts along the direction of an applied electric field. By screening through a first-principles-based database of piezoelectrics for 941 inorganic crystalline compounds, we find that such counterintuitive piezoelectric effect is a general phenomenon. We find that the electric polarization of several ABC ferroelectrics increases with increasing hydrostatic pressure, making them appealing for high-pressure applications. The origin of negative longitudinal piezoelectric response relies on the strong ionic bonds associated with small effective charges and rigid potential energy surfaces.