  CaCu3Ti4O12 (CCTO) ceramics are potential candidates for capacitor
applications due to their large dielectric permittivity (e') values of up to
300 000. The underlying mechanism for the high e' is an internal barrier layer
capacitor (IBLC) structure of insulating grain boundaries (GB) and conducting
grain interiors (bulk). This behaviour is reviewed and discussed in detail. The
origin of the IBLC structure is attributed to a small Cu non-stoichiometry in
nominally insulating CaCu3Ti4O12, which varies between the GBs and bulk. Such
non-stoichiometry effects are studied in detail by analyzing bulk ceramics of
different composition within the ternary CaO-CuO-TiO2 phase diagram using X-ray
diffraction (XRD), scanning electron microscopy (SEM) and impedance
spectroscopy (IS). At least two defect mechanisms are suggested to exist. It is
further shown that the development of the defect mechanisms in CCTO and the
concomitant formation of the IBLC structure strongly depend on the processing
conditions of CCTO ceramic pellets such as the sintering temperature. Nominally
stoichiometric CCTO bulk ceramics sintered at different temperatures are
analyzed using XRD, SEM and IS. The performance of CCTO ceramics for IBLC
applications is controlled by subtle modifications in the compound
stoichiometry that is strongly dependent on the ceramic sintering temperature.
