Atomic-scale insights into ion migration-induced phase transition in halide perovskites

Phase transitions in halide perovskites significantly influence their stability and optoelectronic properties. Revealing the dynamics of octahedral distortion and ion migration during these transitions is vital to understanding their exceptional characteristics. This study reveals the atomic-scale dynamics of phase transitions induced by ion migration in mixed halide perovskites CsPbBrCl₂ using an integrated differential phase contrast scanning transmission electron microscope. Unlike pure perovskites, the orthorhombic-to-cubic transition in mixed halide perovskites involves octahedral distortions and halogen migration. This ion migration facilitates a uniform halogen distribution within the structure, correcting the initially uneven arrangement. The dynamics of octahedral distortions are also examined, with accurate changes in Pb-X bond lengths and distortion angles identified throughout the transition. This cubic phase helps suppress ion migration, as supported by first-principles calculations. These in situ observations enhance the fundamental understanding of octahedral distortions and ion migration in halide perovskites, providing valuable insights for the design of stable perovskite materials.