Degenerate Lattice Instability-Driven Amorphization under Compression in Metal Halide Perovskite CsPbI₃

Due to their good optoelectronic properties and inexpensive cost, halide perovskites have been intensively investigated for photovoltaic applications. Different high-pressure experiments report that they generally undergo the reversible phase transitions between different crystalline phases and between crystalline and amorphous phases under an external pressure. Herein, using first-principles density-functional theory (DFT) and ab-initio molecular dynamics (AIMD) calculations, we investigate the origin of the pressure-induced amorphization in halide perovskite CsPbI₃. We find that the amorphous-like structures obtained from AIMD calculations become more stable than the orthorhombic <i>Pbnm</i> phase above 6.66 GPa, in good agreement with the experimental value (4.44 GPa). Intriguingly, we further find that an imaginary flat band appears in the phonon dispersion of the orthorhombic CsPbI₃ phase across the Brillouin zone at 10 GPa, leading to degenerate lattice instabilities. We demonstrate that these energetically degenerated phonon modes are related to PbI₆ octahedral tilting modes and provide random local distortions, leading to amorphization. Our findings contribute to a greater comprehension of the mechanism behind the pressure-induced amorphization in halide perovskites.