Polarized twin walls in a halide perovskite

Halide perovskites have emerged as promising materials for next-generation photovoltaics, laser sources, and X-ray detectors. There is intense debate as to whether some photoactive halide perovskites exhibit ferroelectric behavior and whether it might be possible to utilize the bulk photovoltaic effect to enhance the performance of halide perovskite solar cells. Here, using low-dose scanning transmission electron microscopy, we identify ferroelastic twin walls in vapor-deposited CsPbI₃ thin films, parallel to {110} and {112}. Remarkably, despite CsPbI₃ being centrosymmetric and nonpolar, we observe directly that Pb atoms shift at {110} twin walls driving a local ferroelectric-like polarization. These polar twin walls form an intrinsic array of nanoscale interfaces, spaced ~30 to 50 nm apart, embedded within the nonpolar perovskite lattice. In contrast, {112} twin walls do not show a polar displacement of Pb but strongly suppress octahedral tilt and off-center Cs atom displacements, revealing a different untapped structural degree of freedom. These findings reveal previously hidden nanoscale polarization in halide perovskite semiconductors, opening opportunities for enhanced conductivity and photovoltaic behavior through domain wall engineering.