Three-Dimensional Perovskite Methylhydrazinium Lead Chloride with Two Polar Phases and Unusual Second-Harmonic Generation Bistability above Room Temperature

Formation of noncentrosymmetric three-dimensional (3D) lead halide perovskites has been a widely sought after goal because the polar structure opens up new vistas to properties of these materials, e.g., improved charge separation for photovoltaics arising from ferroelectric order. Here, we report growth and unique properties of a new highly distorted 3D perovskite, methylhydrazinium lead chloride (CH3NH2NH2PbCl3, MHyPbCl 3 ). This perovskite crystallizes in polar P21 structure at room temperature, which consists of two types of PbCl6 octahedra: one weakly and another strongly deformed. The unusual deformation of every second perovskite layer is forced by the large size of methylhydrazinium cations and the ability of NH2 + terminal groups of methylhydrazinium cations to form coordination bonds with Pb2+ metal centers. On heating, MHyPbCl 3 undergoes a phase transition at 342 K into another polar Pb21 m phase with ordered organic cations. Temperature-resolved second-harmonic generation (TR-SHG) measurements confirm acentricity of both phases and show that second-harmonic response is enhanced for the high-temperature Pb21 m phase. This intriguing property of MHyPbCl 3 has been employed to demonstrate an unprecedented kind of quadratic nonlinear optical switching in which a second-harmonic response is switched between a room-temperature, low-SHG state and a high-temperature, high-SHG state. X-ray diffraction shows that enhancement of polar properties is due to rearrangement of the perovskite’s organic substructure. There is a clear pyrocurrent peak, but switching of the electric polarization could not be observed. Optical studies showed that MHyPbCl 3 is a wide-bandgap material with a bandgap of 3.4 eV (365 nm). At low temperatures, it exhibits weak UV emissions at 362 and 369 nm as well as a strong broadband white emission.