Polarized twin walls in a halide perovskite.

Low-dose scanning transmission electron microscopy reveals ferroelastic twin walls in vapor-deposited CsPbI3 thin films, with Pb shifts at {110} walls driving local ferroelectric-like polarization.

• Why it matters: The work addresses debate over whether photoactive halide perovskites show ferroelectric behavior and whether bulk photovoltaic effects can be harnessed in solar cells.
• What they did: The authors directly image ferroelastic twin walls in CsPbI3 parallel to {110} and {112}, finding polar Pb displacement only at {110} and mapping nanoscale interface spacing of ~30 to 50 nm.
• The result: Polar {110} twin walls form an intrinsic array of nanoscale interfaces embedded in a nonpolar lattice, while {112} walls suppress octahedral tilt and off-center Cs displacements, suggesting domain-wall engineering opportunities.

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