Raman Spectrum of the Organic–Inorganic Halide Perovskite CH₃NH₃PbI₃ from First Principles and High-Resolution Low-Temperature Raman Measurements

Abstract

We investigate the Raman spectrum of the low-temperature orthorhombic phase of the organic–inorganic halide perovskite CH3NH3PbI3, by combining first-principles calculations with high-resolution low-temperature Raman measurements. We find good agreement between theory and experiment and successfully assign each of the Raman peaks to the underlying vibrational modes. In the low-frequency spectral range (below 60 cm–1), we assign the prominent Raman signals at 26, 32, 42, and 49 cm–1 to the Pb–I–Pb bending modes with either Ag or B2g symmetry and the signal at 58 cm–1 to the librational mode of the organic cation. Owing to their significant intensity, we propose that these peaks can serve as clear markers of the vibrations of the [PbI3]− network and of the CH3NH3+ cations in this perovskite, respectively. In particular, the ratios of the intensities of these peaks might be used to monitor possible deviations from the ideal stoichiometry of CH3NH3PbI3.