Anisotropic dielectric behavior of layered perovskite-like Cs₃Bi₂I₉ crystals in the terahertz region

The ternary metal halide perovskites have gradually attracted attention for application in the optoelectronic field, owing to their tunable crystal structure and appropriate bandgap. Lead free Cs3Bi2I9 perovskite, with a 0D layered structure containing molecular [Bi2I9](3-) dimers, exhibits prominent optical and electrical anisotropies. Here, the anisotropic properties of the Cs3Bi2I9 crystals were evaluated using terahertz time-domain spectroscopy (THz-TDS); meanwhile, the effect of phonon vibration on the THz transmission was confirmed using density functional perturbation theory (DFPT). Accordingly, the refractive index and extinction coefficient are estimated using THz-TDS, thanks to the high transmission in the range of 0.2-0.9 THz. The anisotropic refractive index was observed for the Cs3Bi2I9 crystals, and was found to be 3.2-3.7 for the (100) plane (CBI(100)) in contrast to 2.8-3.2 for the (001) plane (CBI(001)). Furthermore, the Lorentz model was employed to extract the dielectric constant of Cs3Bi2I9, in which anisotropy is obviously indicated by the static dielectric constant and the high-frequency dielectric constant. These anisotropic behaviors are determined by the dipole moment, which is attributed to the anisotropic packing density of [Bi2I9](3-) dimers. This study is significant and provides a deeper insight into the anisotropic photoelectric properties of Cs3Bi2I9, thus contributing to the development of metal halide perovskites in the field of optoelectronics.