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

Multilayered Ruddlesden–Popper hybrid lead halide perovskites are an emerging class of soft perovskites that exhibit exceptional linear and nonlinear optical properties as well as ferroelectric and antiferroelectric characteristics. Due to these features, they show immense potential in energy conversion, energy storage, light emission, and photodetector technologies. The structural phase transitions and lattice dynamics of organic cations play key roles in the performance of these perovskites. Herein, we report studies of two bromide perovskites – BA2MHy2Pb3Br10 and iBA2MHy2Pb3Br10 (BA, butylammonium; iBA, iso-butylammonium; and MHy, methylhydrazinium). Our multitechnique approach X-ray diffraction, thermal, dielectric, second harmonic generation (SHG), Raman scattering, and infrared (IR) spectroscopy show that the BA analogue undergoes two distinct structural phase transitions at 372 and 301 K, while the iBA analogue exhibits a more complex sequence of four phase transitions at 364, 262, 223, and 189 K. It turns out that all structural phases are noncentrosymmetric, except for the high-temperature ones. By employing Raman and IR spectroscopy, we reveal that the high-temperature phase transition in both compounds is triggered by ordering of MHy+ cage cations, while the slowing down of the interlayer BA+ and iBA+ cation dynamics plays a major role in the mechanism of the lowest-temperature phase transitions. As such, this study marks the first instance where Raman and IR spectroscopic techniques have been successfully employed to monitor the dynamics of cage and interlayer cations independently within Ruddlesden–Popper perovskites. It also demonstrates that the recently reported excellent polarized photodetection of iBAMHy2Pb3Br10 is primarily related to the polar order of the MHy+ cations. Dielectric studies reveal the presence of the dipolar relaxation process in the iBA2MHy2Pb3Br10 sample below 230 K, associated with motions of iBA+ cations. Both compounds emit luminescence under one- and two-photon excitation, yet the luminescence profiles differ due to the different populations of excitons being excited through linear (365 nm) and nonlinear optical (800 nm) pathways. The luminescence color changes from purplish-blue under the 365 nm excitation to blue under the 800 nm excitation. Overall, our findings show that Ruddlesden–Popper perovskites comprising MHy+ cations may exhibit rich polymorphism, multinoncentrosymmetry, and multiple optical responses (one- and two-photon excited emissions, SHG), which are affected by the type (size and shape) of the interlayer cation and structural phase transitions.