Molecule-Driven Unusual Incommensurate Modulation in a 2D Hybrid Perovskite Ferroelectric

Incommensurate modulation has been extensively investigated in inorganic oxides, representing an emerging frontier in aperiodic materials that enables the discovery of novel quantum phenomena and the manipulation of physical properties, whereas it remains largely unexplored in hybrid perovskites (HPs). Here, we present the driving of successive dynamic rotation of the interlayer dipoles in a two-dimensional HP ferroelectric (DFBZ)2(FA)Pb2I7 (DFBZ is 2,5-difluorobenzylammonium, FA is formamidinium), which enables an unusual incommensurate modulated structure between the paraelectric and ferroelectric phases. Specifically, (DFBZ)2(FA)Pb2I7 exhibits an incommensurate modulated state over a broad temperature range of 313-370 K, and transforms to commensurate modulation at 400-474 K. The incommensurate superstructure at 330 K is ∼6 times larger than its periodic prototype, and its structure can be solved using a (3 + 1)D superspace group with a modulation wavevector of q = (1, 0, 0.1578). The inorganic layers feature a pronounced undulation, while the interlayer organic cations are trapped in distinct rotational states. To the best of our knowledge, (DFBZ)2(FA)Pb2I7 is the first HP ferroelectric that exhibits such a long-range ordered incommensurate modulation. Moreover, its large spontaneous polarization (9.4 μC cm-2) and the near-room temperature ferroelectric-to-incommensurate phase transition result in a giant pyroelectric coefficient of 7.41 μC cm-2 K-1. This unusual incommensurate modulation in HP ferroelectrics offers new insights into the understanding of aperiodic materials and ferroelectric mechanisms.