Unraveling low-temperature structural and dielectric characteristics in lead-free bismuth halide perovskites

Bismuth halide perovskite crystals have garnered a lot of interest lately because of their superior optoelectronic qualities, affordability, and ease of processing. Nonetheless, most of the research has concentrated on their room-temperature characteristics, leaving their low-temperature behavior largely unknown. Here, we have investigated the low-temperature behavior of bismuth halide perovskite crystals to understand their structural and dielectric characteristics. The low-temperature X-ray diffraction pattern revealed a sharp phase transition in MA₃Bi₂I₉, a gradual phase transition in FA₃Bi₂I₉ and a ferroelastic phase transition in Cs₃Bi₂I₉. These structural characteristics are directly mirrored and linked to the material's dielectric function. The dielectric properties of these crystals revealed that the features of the A-site cations, rather than the inorganic bioctahedral cage, have the greatest influence on the behavior of these materials. The presence of microscopic dipoles has a significant influence on the macroscopic dielectric constant, resulting in a dielectric constant hierarchy. It was found that MA₃Bi₂I₉ exhibits a higher value of dielectric constant than FA₃Bi₂I₉ and Cs₃Bi₂I₉, due to the rotational dynamics of MA⁺ ions with large dipole moments. The findings of this study would contribute to the advancement of perovskite-based optoelectronics by providing a detailed understanding of the low-temperature properties of bismuth halide perovskites with A-site cation variation.