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Interfaces such as grain boundaries (GBs) and heterointerfaces (HIs) are known to play a crucial role in structure-property relationships of polycrystalline materials. While several methods have been used to characterize such interfaces, advanced transmission electron microscopy (TEM) and scanning TEM (STEM) techniques have proven to be uniquely powerful tools, enabling quantification of atomic structure, electronic structure, chemistry, order/disorder, and point defect distributions below the atomic scale. This review focuses on recent progress in characterization of polycrystalline oxide interfaces using S/TEM techniques including imaging, analytical spectroscopies such as energy dispersive X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS) and scanning diffraction methods such as precession electron nano diffraction (PEND) and 4D-STEM. First, a brief introduction to interfaces, GBs, HIs, and relevant techniques is given. Then, experimental studies which directly correlate GB/HI S/TEM characterization with measured properties of polycrystalline oxides are presented to both strengthen our understanding of these interfaces, and to demonstrate the instrumental capabilities available in the S/TEM. Finally, existing challenges and future development opportunities are discussed. In summary, this article is prepared as a guide for scientists and engineers interested in learning about, and/or using advanced S/TEM techniques to characterize interfaces in polycrystalline materials, particularly ceramic oxides. 

Abstract Deploying energy storage and carbon capture at scale is hindered by the substantial endothermic penalty of decomposing CaCO₃to CaO and CO₂, and the rapid loss of CO₂absorption capacity by CaO sorbent particles due to sintering at the high requisite decomposition temperatures. The decomposition reaction mechanism underlying sorbent deactivation remains unclear at the atomic level and nanoscale due to past reliance on postmortem characterization methods with insufficient spatial and temporal resolution. Thus, elucidating the important CaCO₃decomposition reaction pathway requires direct observation by time‐resolved (sub‐)nanoscale methods. Here, chemical and structural dynamics during the decomposition of CaCO₃nanoparticles to nanoporous CaO particles comprising high‐surface‐area CaO nanocrystallites are examined. Comparing in situ transmission electron microscopy (TEM) and synchrotron X‐ray diffraction experiments gives key insights into the dynamics of nanoparticle calcination, involving anisotropic CaCO₃thermal distortion before conversion to thermally dilated energetically stable CaO crystallites. Time‐resolved TEM uncovered a novel CaO formation mechanism involving heterogeneous nucleation at extended CaCO₃defects followed by sweeping reaction front motion across the initial CaCO₃particle. These observations clarify longstanding, yet incomplete, reaction mechanisms and kinetic models lacking accurate information about (sub‐)nanoscale dynamics, while also demonstrating calcination of CaCO₃without sintering through rapid heating and precise temperature control.