Evidence of Cu deficiency:: A key point for the understanding of the mystery of the giant dielectric constant in CaCu3Ti4O12

The compositions of the grains and the grain boundaries in CaCu3Ti4O12 (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.