Abstract
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, MHyPbCl3). 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, MHyPbCl3 undergoes a phase transition at 342 K into another polar Pb21m 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 Pb21m phase. This intriguing property of MHyPbCl3 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 MHyPbCl3 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.
Highlights
Hybrid organic−inorganic compounds have been the subject of intense studies because their diverse structural and chemical variability offers unlimited opportunities for tuning their physical and chemical properties by chemical modification of the organic and/or inorganic parts
By comparison of the integral intensity of the secondharmonic generation (SHG) signal to the one collected for the Kurtz−Perry technique. Potassium dihydrogen phosphate (KDP) powder of the same particle size (Figure S8), we found that phase II of MHyPbCl3 offers an SHG response approximately equal to 0.03 that of KDP
We report the synthesis of a new all-noncentrosymmetric 3D perovskite, methylhydrazinium lead chloride
Summary
Hybrid organic−inorganic compounds have been the subject of intense studies because their diverse structural and chemical variability offers unlimited opportunities for tuning their physical and chemical properties by chemical modification of the organic and/or inorganic parts. In contrast to the temperature-resolved SHG results, in which the phase transition had a clear effect on the intensity of the produced second harmonic of radiation, the observed dielectric anomaly leads to a rather subtle change in the ε′ value. The acentricity of both phases and the observed pyroelectric response could indicate a proper ferroelectric behavior of MHyPbCl3 This phenomenon is occasionally encountered in hybrid compounds,[50,51] the majority of such compounds are less attractive from the applicational standpoint for improper ferroelectrics.[52] To check for a proper ferroelectric response, we measured the electric-field dependence of the electric polarization at 323 K of a single-crystal sample The chromaticity coordinates (x, y) of the white emission change from warmer (0.33, 0.40) at 120 K to colder (0.30, 0.33) at 150 K
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