Ultra-Low Frequency Raman and Infrared Vibrational Modes in Metal-Halides

Lattice dynamics are critical to photovoltaic material performance, governing dynamic disorder, hot-carrier cooling, charge-carrier recombination and transport. Soft metal-halide perovskites exhibit particularly intriguing dynamics, with Raman spectra exhibiting an unusually broad low-frequency response whose origin is still much debated. Here, we utilise ultra-low-frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination of the vibrational response for a wide range of metal-halide perovskites and other metal-halides: FAPbI3, MAPbIxBr3-x, CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. Stark differences between Raman and IR spectra are presented in this work, wherein such central Raman response is clearly visible in Raman spectra, but is seemingly absent in IR spectra. After careful assessment from our data and literature, we are able to rule out extrinsic defects, octahedral tilting, cation lone pairs and ‘liquid-like’ Boson peaks as causes of the debated central Raman peak. We discuss different reduction schemes to better represent the vibrational density of states from different spectroscopic measurements. Further, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose-Einstein statistics towards low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.<br/><br/>Reference: Lim et al. ACS Energy Lett. 2024, 9, 4127−4135