Evidence of the internal domains for inducing the anomalously high dielectric constant of CaCu3Ti4O12

The delicate self-intertwined domain structures inside the grains of the polycrystalline CaCu3Ti4O12 (CCTO) were discovered by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), which provided a direct clue in understanding the mystery of the huge dielectric response of this intriguing material. Domain morphologies with and without terrace and ledge were observed by SEM. The detailed domain structures were further examined by HRTEM, and it was found that they contained strained and unstrained domains. The strained domains essentially are constructed by various dimensional superlattices. The coherent domain boundaries were considered to be strained, composition-disordered, and insulating. A microstructural model and its equivalent circuit based on a series connection with semiconducting domain interior, insulating domain boundary, and insulating grain boundary have been established, which not only solve the contradictory explanation of the dielectric response between polycrystalline and single-crystal CCTO but also provide a reasonable interpretation of the experimental results related to enhancement of the dielectric responses of CCTO.