Origin of Phase Transitions in Inorganic Lead Halide Perovskites: Interplay between Harmonic and Anharmonic Vibrations

Inorganic lead halide perovskites (ILHPs) exhibit a series of phase transitions, and stabilization of the phases with desirable optoelectronic properties remains a major challenge. However, the intrinsic origins of structural instabilities in CsPbX 3 (X = Br, I) are still elusive. Herein, the important role of harmonic and anharmonic vibrations in influencing thermodynamic fluctuations of ILHPs was revealed, through combined lattice dynamics and multiphonon theory calculations, and verified by diffraction experiments. Our results demonstrate that the transition between δ- and γ-CsPbI 3 is driven by harmonic vibrations, unveiling the mysterious mechanism for stabilizing γ-CsPbI 3 via applying strain. Moreover, the successive transitions from the α- to β- and β- to γ-phases of CsPbX 3 are driven by anharmonic vibrations. These structural dynamics are strongly coherent with the phonon diffuse scattering, substantially affecting the thermal conductivity and carrier relaxation. This work provides guidelines for maintaining favorable ILHP phases through delicately manipulating their lattice dynamics.