Large Piezoelectric Response in a Metal-Free Three-Dimensional Perovskite Ferroelectric.

Metal-free perovskites with light weight and eco-friendly processability have received great interest in recent years due to their superior physical features in ferroelectrics, X-ray detection, and optoelectronics. The famous metal-free perovskite ferroelectric MDABCO-NH 4 -I 3 (MDABCO = N -methyl- N '-diazabicyclo[2.2.2]octonium) has been demonstrated to exhibit excellent ferroelectricity comparable to that of inorganic ceramic ferroelectric BaTiO 3 , such as large spontaneous polarization and high Curie temperature (Ye et al. Science 2018 , 361 , 151). However, piezoelectricity as a vitally important index is far from enough in the metal-free perovskite family. Here, we report the discovery of large piezoelectric response in a new metal-free three-dimensional perovskite ferroelectric NDABCO-NH 4 -Br 3 (NDABCO = N -amino- N '-diazabicyclo[2.2.2]octonium) by replacing the methyl group of MDABCO with the amino group. Besides the evident ferroelectricity, strikingly, NDABCO-NH 4 -Br 3 shows a large d 33 of 63 pC/N more than 4 times that of MDABCO-NH 4 -I 3 (14 pC/N). The d 33 value is also strongly supported by the computational study. To the best of our knowledge, such a large d 33 value ranks the highest among the documented organic ferroelectric crystals to date and represents a major breakthrough in metal-free perovskite ferroelectrics. Combined with decent mechanical properties, NDABCO-NH 4 -Br 3 is expected to be a competitive candidate for medical, biomechanical, wearable, and body-compatible ferroelectric devices.