Unconventional ferroelectricity, robust at reduced nanoscale sizes, exhibited by hafnia-based thin-films presents tremendous opportunities in nanoelectronics. However, the exact nature of polarization switching remains controversial. Here, we investigate epitaxial Hf0.5Zr0.5O2(HZO) capacitors, interfaced with oxygen conducting metals (La0.67Sr0.33MnO3, LSMO) as electrodes, using atomic resolution electron microscopy while in situelectrical biasing. By direct oxygen imaging, we observe reversible oxygen vacancy migration from the bottom to the top electrode through HZO and reveal associated reversible structural phase transitions in the epitaxial LSMO and HZO layers. We follow the phase transition pathways at the atomic scale and identify that these mechanisms are at play both in tunnel junctions and ferroelectric capacitors switched with sub-millisecond pulses. Our results unmistakably demonstrate that oxygen voltammetry and polarization switching are intertwined in these materials.

Metadata: File types: 1. .emd for electron microscopy data 2. .h5 for synchrotron data 3. .opg for analysis of electrical data, and plots from quantitative electron microscopy and X-ray diffraction

.emd files: Nomenclature Fig #- type of images-magnification.emd Abbrevations (general) STEM: Scanning Transmission Electron Microscope Abbrevations (Image types) DPC: Differential Phase Contrast HAADF: High angle annular dark field DF4: 4’ Dark field detector image (roughly corresponds to annular bright field) SI: Spectrum Image EDS: Energy Dispersive Spectrum Example: Fig 1b DPC HAADF DF4 4.9 Mx.emd means this is Fig 1B presented in the paper, and the file contains DPC (Differential phase contrast), HAADF (High angle annular dark field) and DF4 (Annular Bright field) images each obtained at 4.9 Mx magnification.

Multiple datasets from the same figure # appear in a folder named with the Figure #. Location of the dataset used in the figure is represented as right/left.

.h5 files Nomenclature: By themselves #.hdf5 files are metadata files containing information about the intensities on the 2D merlin detector at various 2-theta (angle between x-ray source and detector). # represents the experiment number in the Max-IV beam line. All the synchrotron data used in this work and corresponding python codes for analysis are stored in separate folders, each folder named as the figure in which this data is used. For example, folder named Fig S10a consists of .h5 data file, .h5 mask file, and the python code used to analyse this data to generate what was presented in Fig S10a

.opj files: Nomenclature Figure # - data explanation.opj Example : Fig 4c -collated P-E-T data.opj means Figure #4c as presented in the paper, and the origin file consists of Polarization-Electric field-temperature data and its analysis used to generate Fig 4c.

Codes for Microscopy data: Python and Matlab codes used for analysis of microscopy data are in the folder: “Codes for TEM” Files are named as: Fig numbers- explanation of the code .m/txt. Example Fig 3b-S6a and b python code for oxygen displacement.txt means this code was used to analyze data in Figs 3b, S6a and S6b published in the paper. This code gives the oxygen displacement in hafnia layer.