Deciphering the nature of anharmonicity in CH₃NH₃PbI₃ by THz range Raman crystallography

Methylammonium lead iodide (CH₃NH₃PbI₃ or MAPI) shows many seemingly contradictory characteristics, like low diffusion but high carrier lifetime, small Urbach energy but defect tolerance etc., which are intimately connected to its anharmonic structural dynamics. Therefore, the fundamental understanding of the anharmonicity in MAPI is critical for harnessing its full potential in future optoelectronic devices. We combine THz-range Raman crystallography and ab initio computation to elucidate the microscopic mechanisms governing the strongly anharmonic structural dynamics in MAPI. Correlation between observed Raman intensity modulations with PbI₆ octahedral tilting patterns provides a direct evidence of highly anharmonic and anisotropic nature of such low-frequency vibrations. The anisotropy and anomalous broadening of the Raman features throughout the tetragonal phase is manifested by athermal anharmonicity. Conversely, thermally induced all-axes relaxational rotation causes anharmonicity and consequently, isotropy in cubic phase. Our study delineates the anharmonic structural dynamics of the technologically relevant tetragonal phase MAPI and offer insights to utilize anharmonicity to engineer favorable optoelectronic properties in materials with long term stability.