Multinoncentrosymmetric Two-Dimensional Trilayered\nLead Bromide Perovskites with Methylhydrazinium Cations: Lattice Dynamics,\nPhase Transitions, Dielectric Response, and Optical Properties

Multilayered Ruddlesden–Popper\nhybrid lead halide perovskites\nare an emerging class of soft perovskites that exhibit exceptional\nlinear and nonlinear optical properties as well as ferroelectric and\nantiferroelectric characteristics. Due to these features, they show\nimmense potential in energy conversion, energy storage, light emission,\nand photodetector technologies. The structural phase transitions and\nlattice dynamics of organic cations play key roles in the performance\nof these perovskites. Herein, we report studies of two bromide perovskites\n– BA<sub>2</sub>MHy<sub>2</sub>Pb<sub>3</sub>Br<sub>10</sub> and iBA<sub>2</sub>MHy<sub>2</sub>Pb<sub>3</sub>Br<sub>10</sub> (BA,\nbutylammonium; iBA, iso-butylammonium; and MHy, methylhydrazinium).\nOur multitechnique approach [X-ray diffraction, thermal, dielectric,\nsecond harmonic generation (SHG), Raman scattering, and infrared (IR)\nspectroscopy] show that the BA analogue undergoes two distinct structural\nphase transitions at 372 and 301 K, while the iBA analogue exhibits\na more complex sequence of four phase transitions at 364, 262, 223,\nand 189 K. It turns out that all structural phases are noncentrosymmetric,\nexcept for the high-temperature ones. By employing Raman and IR spectroscopy,\nwe reveal that the high-temperature phase transition in both compounds\nis triggered by ordering of MHy<sup>+</sup> cage cations, while the\nslowing down of the interlayer BA<sup>+</sup> and iBA<sup>+</sup> cation\ndynamics plays a major role in the mechanism of the lowest-temperature\nphase transitions. As such, this study marks the first instance where\nRaman and IR spectroscopic techniques have been successfully employed\nto monitor the dynamics of cage and interlayer cations independently\nwithin Ruddlesden–Popper perovskites. It also demonstrates\nthat the recently reported excellent polarized photodetection of iBAMHy<sub>2</sub>Pb<sub>3</sub>Br<sub>10</sub> is primarily related to the\npolar order of the MHy<sup>+</sup> cations. Dielectric studies reveal\nthe presence of the dipolar relaxation process in the iBA<sub>2</sub>MHy<sub>2</sub>Pb<sub>3</sub>Br<sub>10</sub> sample below 230 K,\nassociated with motions of iBA<sup>+</sup> cations. Both compounds\nemit luminescence under one- and two-photon excitation, yet the luminescence\nprofiles differ due to the different populations of excitons being\nexcited through linear (365 nm) and nonlinear optical (800 nm) pathways.\nThe luminescence color changes from purplish-blue under the 365 nm\nexcitation to blue under the 800 nm excitation. Overall, our findings\nshow that Ruddlesden–Popper perovskites comprising MHy<sup>+</sup> cations may exhibit rich polymorphism, multinoncentrosymmetry,\nand multiple optical responses (one- and two-photon excited emissions,\nSHG), which are affected by the type (size and shape) of the interlayer\ncation and structural phase transitions.