Direct Observation of Noncollinear Ferrielectricity in a Two-Dimensional Hybrid Germanium Perovskite.

The discovery of ferrielectricity, which can be termed as an antiferroelectric order with an uncompensated dipole, has created a new wonderland for exploring and coupling multiple dipolar orders. However, limited by stringent symmetry and ambiguous multiple definitions, experimental visualization of ferrielectricity is still considerably rare in crystalline solids. Herein, the distinctive noncollinear ferrielectricity has been experimentally visualized in a 2D hybrid germanium perovskite, (butylammonium)GeI. From polarization-electric field loops, the ferroelectric polarization in the out-of-plane direction and antiferroelectricity in the in-plane direction have been demonstrated in its single crystal. As temperature increases, (butylammonium)GeI undergoes the consecutive transition of ferroelectric-ferrielectric-ferroelectric-paraelectric, involving symmetry transformation. Temperature-dependent crystal structure analysis and theoretical calculations reveal that the dynamic organic cations and ion displacement promoted by the stereoactive lone pair electrons compete, leading to the distinctive ferrielectric order. This work offers fascinating opportunities for exploring ferrielectrics integrating multiple dipolar orders, highlighting their distinctive functionalities and promising applications.