Surface Ferroelectricity in Lead‐Free Nonferroelectric Chiral Perovskites Beyond Neumann's Principle

Abstract

ABSTRACT According to Neumann's Principle, the chiral‐polar photovoltaic effect (CPPE) is restricted to crystals in one of the five chiral ferroelectric space groups—based on the assumption of perfect long‐range periodicity and bulk symmetry. However, this framework overlooks symmetry breaking at crystal boundaries. Herein, we report the emergence of surface ferroelectricity in a one‐dimensional chiral nonferroelectric perovskite, R / S ‐MPZSbBr 5 (MPZ = 2‐methylpiperazine), achieved through interlayer antiparallel alignment of polar Sb–Br chains. The bulk structure adopts a nonferroelectric phase with alternating polar domains, resulting in net‐zero polarization. Yet, at the (001) surface, the breaking of continuous translational symmetry gives rise to local ferroelectricity. This surface‐driven polarization, coupled with the intrinsic chirality of the crystal, enables a localized CPPE. Moreover, the synergy between surface ferroelectricity and pyro‐photovoltaic effects significantly enhances the performance of self‐powered circularly polarized light (CPL) detection. Our work reveals the limitations of Neumann's Principle at crystal interfaces and opens new avenues for engineering ferroelectricity and chiral optoelectronics in nonferroelectric systems.