Ammonium Sulfate Structure-Type Hybrid Metal Halide Ferroelectric with Giant Uniaxial Spontaneous Strain

Hybrid metal halide ferroelectrics have attracted intense attention due to their simple synthesis, mechanical flexibility, and light weight. There have been many metal halide ferroelectrics discovered thus far; however, their structural design is heavily centralized on the perovskite prototype which largely hampers the expansion of the landscape of these emerging molecular systems. Herein, we report the synthesis and structure of a new hybrid metal halide ferroelectric, (TMS)2CdI4 (TMS = trimethylsulfonium), which has an ammonium sulfate-type structure. This material undergoes a first-order paraelectric to ferroelectric transition which is driven by the order–disorder of the TMS cations similar to the scenario in ammonium sulfate. Notably, this transition involves a giant uniaxial spontaneous strain up to 13.1% which has very rarely been observed in molecular ferroelectrics. The intrinsic ferroelectricity of (TMS)2CdI4 was made evident by observing switchable domains in the fabricated thin films via piezoresponse force microscopy. This work implies that there is plenty of space in designing the structures of hybrid metal halide ferroelectrics beyond the overemphasized perovskite prototype.