In Situ Transmission Electron Microscopy Visualization of Electric-Field-Induced Phase Transitions at the Morphotropic Phase Boundary in Hf 0.5 Zr 0.5 O 2

, HZO) thin films. Here, we use in situ transmission electron microscopy (TEM) to uncover the nanoscale mechanism of field-induced phase evolution in ultrathin HZO films at the morphotropic phase boundary (MPB), directly visualizing oxygen vacancy migration and its correlation with the transformation from the nonpolar tetragonal to polar orthorhombic phase. Our in situ TEM setup applied sub-100 μs bipolar voltage pulses, mimicking real device operation while allowing the detection of the subtle changes induced by such short pulses. Unsupervised machine learning analysis of electron energy-loss spectroscopy spectrum images (EELS-SIs) revealed distinct spectral features associated with local structural evolution, with quantitative results confirming oxygen-deficient regions aligned with orthorhombic phase formation. Unlike conventional TEM studies confined to a few nanoscale domains, this approach enables film-scale interpretation of phase evolution, capturing broader trends beyond isolated observations. Concurrent oxygen content changes in the TiN electrode further indicate active vacancy exchange between HZO and TiN under bias. These findings directly link oxygen vacancy dynamics to polarization switching, offering critical guidance for stabilizing ferroelectric phases and advancing next-generation memory and logic devices.