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The compositions of the grains and the grain boundaries in CaCu 3 Ti 4 O 12 (CCTO) have been characterized by a high-resolution field emission transmission electron microscope equipped with a high-angle annular-dark-field detector and energy-dispersive X-ray spectroscopy (EDS). The excess ions and their clusters at grain boundaries led to the observed strain contrast. These ions are further clarified as Cu ions by Z-contrast (Z: atomic number) image and EDS. The quantitative EDS analyses further indicate that the grain boundaries of CCTO are Cu rich and the grains are Cu deficient. The Ca-or Ti-rich amorphous phases were observed at corners or edges of the grains of CCTO.

In this study, the relationship between the grain‐boundary composition and varistor properties were investigated using scanning electron microscopy, transmission electron microscopy, energy dispersion spectroscopy, and X‐ray photoelectric spectroscopy (XPS). It was found that the CaCu3Ti4O12 (CCTO) sample did not exhibit varistor behavior when the compositions of grain boundary and grain were nearly the same before discontinuous grain growth. The CCTO sample exhibited varistor behavior after the occurrence of discontinuous grain growth. After discontinuous grain growth, copper (Cu)‐rich secondary phase appeared at the grain boundaries. The XPS results reveal that Cu+ is the major content of the second phases. The formation of negatively charged species, , or, existing at the grain boundaries due to the oxidation of the secondary phase during cooling after sintering can act as acceptors, forming double Schottky barriers with the n‐type semiconductor grains.

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