  Using transmission electron microscopy (TEM) we studied CaCu3Ti4O12, an
intriguing material that exhibits a huge dielectric response, up to kilohertz
frequencies, over a wide range of temperature. Neither in single crystals, nor
in polycrystalline samples, including sintered bulk- and thin-films, did we
observe the twin domains suggested in the literature. Nevertheless, in the
single crystals, we saw a very high density of dislocations with a Burger
vector of [110], as well as regions with cation disorder and planar defects
with a displacement vector 1/4[110]. In the polycrystalline samples, we
observed many grain boundaries with oxygen deficiency, in comparison with the
grain interior. The defect-related structural disorders and inhomogeneity,
serving as an internal barrier layer capacitance (IBLC) in a semiconducting
matrix, might explain the very large dielectric response of the material. Our
TEM study of the structure defects in CaCu3Ti4O12 supports a recently proposed
morphological model with percolating conducting regions and blocking regions.
