Halide ion phase separation is a barrier to the application of mixed-halide perovskites whereby the presence of large populations of photogenerated or injected carriers causes undesirable changes in the local band gap. We investigate the mechanism of phase separation in CsPbI_xBr_3-x perovskite single crystals driven by light. The phase separation process and its dynamics are visualized at the nanometer scale at cryogenic temperatures using in situ scanning transmission electron microscopy and cathodoluminescence. Combined with phase field modeling, which accounts for the coupling between electronic and chemical driving forces, our observations point to a spinodal decomposition mechanism in which both the amplitude of composition fluctuation and the characteristic length scale grow non-linearly with time. Our findings provide microscopic insights that can assist in further engineering mixed-halide perovskites either for stability or for intentional programming of the local halide ion composition, opening pathways to a wide range of applications.

卤化物离子相分离是混合卤化物钙钛矿应用的障碍，大量光生或注入载流子的存在会导致局部带隙发生不良变化。_{我们研究了光驱动的 CsPbI x} Br _3-x钙钛矿单晶的相分离机制。相分离过程及其动力学在低温下使用原位扫描透射电子显微镜在纳米尺度上可视化阴极发光。结合考虑电子和化学驱动力之间耦合的相场建模，我们的观察指出了旋节线分解机制，其中成分波动的幅度和特征长度尺度都随时间非线性增长。我们的研究结果提供了微观见解，可以帮助进一步设计混合卤化物钙钛矿以提高稳定性或有意规划局部卤化物离子组成，从而为广泛的应用开辟道路。