Observing Phase Transition Dynamics in Halide Perovskite Nanowires via in Situ 4D-STEM

Phase transitions in halide perovskites critically influence their optoelectronic performance and stability, yet the nanoscale pathways by which structural transformations proceed remain elusive because of their soft ionic lattice and electron-beam sensitivity. Here, we combine in situ heating with four-dimensional scanning transmission electron microscopy (4D-STEM) to investigate the phase-transition dynamics in single CsPbBrxI3-x nanowires, from the non-perovskite to the perovskite phase, with nanometer spatial and millisecond temporal resolution. We directly visualize and track the real-time propagation of the perovskite phase front along a nanowire, while mapping local crystallographic order and diffuse scattering during the transition. The emergence of amorphous diffraction rings indicates a breakdown of long-range crystalline order, and with molecular dynamics simulations, we can conclude that local atomic coordination is preserved. More broadly, this work establishes a general framework for resolving phase transition pathways in beam-sensitive materials beyond halide perovskites, with implications for stabilizing functional phases for future applications.