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

Abstract

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-NH₄-I₃ (MDABCO = <i>N</i>-methyl-<i>N</i>'-diazabicyclo[2.2.2]octonium) has been demonstrated to exhibit excellent ferroelectricity comparable to that of inorganic ceramic ferroelectric BaTiO₃, such as large spontaneous polarization and high Curie temperature (Ye et al. <i>Science</i> <b>2018</b>, <i>361</i>, 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-NH₄-Br₃ (NDABCO = <i>N</i>-amino-<i>N</i>'-diazabicyclo[2.2.2]octonium) by replacing the methyl group of MDABCO with the amino group. Besides the evident ferroelectricity, strikingly, NDABCO-NH₄-Br₃ shows a large <i>d</i>₃₃ of 63 pC/N more than 4 times that of MDABCO-NH₄-I₃ (14 pC/N). The <i>d</i>₃₃ value is also strongly supported by the computational study. To the best of our knowledge, such a large <i>d</i>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-NH₄-Br₃ is expected to be a competitive candidate for medical, biomechanical, wearable, and body-compatible ferroelectric devices.