Geometrical Designing of a Soft‐Layered Halide Perovskite Improper Photoferroelectric for Boosted Photostriction

Abstract Halide perovskite photoferroelectrics, materials possessing both photosensitive and ferroelectric properties, have motivated tremendous attention in the past decade owing to their distinguishing bulk photovoltaic effect (BPVE) and cross‐coupled functionalities. Despite the great achievements, the BPVE coupled functionalities are still limited by the geometric origin contradictory of internal order parameters. Herein, by exploiting a geometrical design approach, a soft layered halide perovskite improper photoferroelectric (2FEA)₂(EA)₂Pb₃Br₁₀ (1, where 2FEA is 2,2‐difluoroethylamonium and EA is ethylammonium) has been developed. Crystallographic investigations reveal that the incorporation of large‐size EA cations significantly induces a nonpolar lattice distortion of the inorganic semiconducting framework, which gives rise to a striking improper ferroelectricity. In the soft crystal lattices, driven by the subtle competition and coupling between photoinduced carriers and inorganic lattice distortions, single crystals of 1 display a giant cross‐coupled photostriction with a considerable photostrictive coefficient up to 4.89 × 10⁻⁶ m² W⁻¹, which is larger than the traditional semiconductors and ferroelectrics. This work offers an attractive route to the design of new improper photoferroelectric materials and promotes their further applications in photoelectronic devices.