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

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-MPZSbBr₅ (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.