Large piezoelectric response in a Jahn-Teller distorted molecular metal halide.

Piezoelectric materials convert between mechanical and electrical energy and are a basis for self-powered electronics. Current piezoelectrics exhibit either large charge (d 33 ) or voltage (g 33 ) coefficients but not both simultaneously, and yet the maximum energy density for energy harvesting is determined by the transduction coefficient: d 33 *g 33 . In prior piezoelectrics, an increase in polarization usually accompanies a dramatic rise in the dielectric constant, resulting in trade off between d 33 and g 33 . This recognition led us to a design concept: increase polarization through Jahn-Teller lattice distortion and reduce the dielectric constant using a highly confined 0D molecular architecture. With this in mind, we sought to insert a quasi-spherical cation into a Jahn-Teller distorted lattice, increasing the mechanical response for a large piezoelectric coefficient. We implemented this concept by developing EDABCO-CuCl 4 (EDABCO = N-ethyl-1,4-diazoniabicyclo[2.2.2]octonium), a molecular piezoelectric with a d 33 of 165 pm/V and g 33 of ~2110 × 10 -3  V m N -1 , one that achieved thusly a combined transduction coefficient of 348 × 10 -12  m 3  J -1 . This enables piezoelectric energy harvesting in EDABCO-CuCl 4 @PVDF (polyvinylidene fluoride) composite film with a peak power density of 43 µW/cm 2 (at 50 kPa), the highest value reported for mechanical energy harvesters based on heavy-metal-free molecular piezoelectric.