Bilayered Two-Dimensional Hybrid Perovskite with Switchable Dielectric Properties, Persistent Disorder, and Narrow-Band Emission.

Two-dimensional (2D) multilayered lead halide perovskites have shown great potential for application in optoelectronics. Here, single crystals of 2D perovskite IPA2FAPb2Br7 (IPA = isopropylammonium, FA = formamidinium) were grown, and the phase transition mechanism, vibrational, dielectric, and optical properties were investigated. IPA2FAPb2Br7 crystallizes in the I4/mmm space group and undergoes two phase transitions at 303 and 136 K. The higher-temperature one involves only partial structural ordering and is accompanied by a distinct dielectric anomaly, enabling dielectric switching. In contrast, the low-temperature transition has mainly displacive character, and Arrhenius extrapolation suggests that FA+ dynamics may persist down to approximately 70-80 K. Consequently, IPA2FAPb2Br7 acts as a dielectric switch with persistent disorder, which allows the compound to relax even at low temperatures and enables continuous modulation of its optoelectronic properties. Photoluminescent (PL) studies show that IPA2FAPb2Br7 exhibits narrowband emission with a maximum near 435 nm that was tentatively assigned to localized excitons, as its temperature dependence does not follow the bandgap evolution. Overall, these findings identify IPA2FAPb2Br7 as a promising light-emitting material and dielectric switch and highlight the importance of complementary temperature-dependent PL and transmission studies for understanding the optoelectronic properties of multilayered perovskites and the origin of PL.