The ultrafast charging/discharging rates, high voltage endurance, superior reliability and broad operating temperature [1,2] of dielectric-based electrostatic capacitors make them promising for the design of electrical power systems such as portable/wearable electronics and collection of fluctuating energy sources. Aiming at further improving the recoverable energy density Ue = ∫EdP, in situ investigation of structure-property relationship provides an effective strategy for understanding the structural features that are responsible for the energy storage behavior. However, the ultrafast charging and discharging processes, completed in milliseconds or less in traditional parallel-plate capacitors, bring great challenges to capture the nature of the process, i.e., the transient transition between different phase states [3,4].
Here, we report a time- and atomic-resolution transmission electron microscopy study of the energy storage process in PbZrO3 by using electron beam irradiation as an external stimulus. Through imaging light oxygen and heavy atoms using negative spherical aberration imaging (NCSI) technique, our measurement shows that the unit-cell volume shrinks and then expands during the orthorhombic (AFEO)-to-monoclinic (FEM) and FEM-to-rhombohedral (FER) phase transitions. Specifically, quantitative tracking of oxygen octahedral rotation reveals an unconventional FE-ferrodistortive transient phase [5], which is characteristic of Pb antiparallel displacements and a cycloidal polarization order (Fig. 1A-C), and bridges the AFEO and FEM phases. In oxygen-and-Pb deficient PbZrO3 [6], our in situ atomic-resolution study further reveals point-defect-induced unit-cell-wise energy storage pathway (Fig. 1D-I). These findings unveil a new territory for exploring novel ferrodistortive phases in nonpolar dielectric materials [7] and provide a straightforward approach for visualizing energy storage process in defect-engineered dielectric ceramics.

References
[1] Hao Pan et al., Science 365, 578 (2019).
[2] J. Li et al., Nat. Mater. 19, 999 (2020).
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[4] H. He, X. Tan, Phys. Rev. B 72, 024102 (2005).
[5] X.-K. Wei, C. L. Jia, H. C. Du, K. Roleder, J. Mayer, R. E. Dunin-Borkowski, Adv. Mater. 32, 1907208 (2020).
[6] X.-K. Wei, C. L. Jia, K. Roleder, R. E. Dunin‐Borkowski, J. Mayer, Adv. Funct. Mater.  31, 2008609 (2021).
[7] X.-K. Wei, R. E. Dunin-Borkowski, J. Mayer, Materials 14, 7854 (2021).