<title>Abstract</title> Understanding the atomic scale structural dynamics of phase transformations is crucial for developing new materials and tailoring the properties of the materials. However, many materials are obtained as polycrystalline powders with large unit cells and/or complex structures, making it challenging to investigate detailed structural changes using conventional X-ray diffraction techniques. Here, we employ time-resolved 3D electron diffraction (3D ED) to unravel the topotactic reactions and tra…