Multiferroic Orders and Piezoelectric Sensing in an Organic-Inorganic Bismuth Halide

Multiple ferroic orders integrated into a single material pose great potential for next-generation sensing, memory and switching devices. Organic-inorganic hybrid materials (OIHMs) have emerged as promising candidates due to their synergistic combination of functional organic cations and inorganic frameworks. However, obtaining multiferroic materials has long been a continuous challenge, and bismuth-based OIHM multiferroics remain a blank to date. Here, we report a multiferroic OIHM (DMPA)₃Bi₂Br₉ (DMPA = N,N-dimethylisopropylamine), which couples ferroelectric and ferroelastic properties for the first time among bismuth-based OIHMs. Driven by dynamic cation reorientation, (DMPA)₃Bi₂Br₉ undergoes multiple symmetry-breaking processes, enabling $\overline{3}$ mFm-type ferroelectric and 3mFm-type multiferroic phase transitions as well as exceptional four-state SHG transition behaviors. Benefiting from these attributes, (DMPA)₃Bi₂Br₉ crystals achieve a notable piezoelectric coefficient d₃₃ of 35 pC/N, enabling its flexible composite with polyurethane to show excellent piezoelectric sensing responses for pose recognition. This work not only expands the family of multiferroic materials but also underscores their promise for next-generation flexible electronics and switching devices.