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

Inorganiclead halide perovskites (ILHPs) exhibit a series of phasetransitions, and stabilization of the phases with desirable optoelectronicproperties remains a major challenge. However, the intrinsic originsof structural instabilities in CsPbX3 (X = Br, I) are stillelusive. Herein, the important role of harmonic and anharmonic vibrationsin influencing thermodynamic fluctuations of ILHPs was revealed, throughcombined lattice dynamics and multiphonon theory calculations, andverified by diffraction experiments. Our results demonstrate thatthe transition between & delta;- and & gamma;-CsPbI3 is drivenby harmonic vibrations, unveiling the mysterious mechanism for stabilizing & gamma;-CsPbI3 via applying strain. Moreover, the successivetransitions from the & alpha;- to & beta;- and & beta;- to & gamma;-phasesof CsPbX3 are driven by anharmonic vibrations. These structuraldynamics are strongly coherent with the phonon diffuse scattering,substantially affecting the thermal conductivity and carrier relaxation.This work provides guidelines for maintaining favorable ILHP phasesthrough delicately manipulating their lattice dynamics.