Influence of high levels of Nb and Ti doping on the dielectric properties of CaCu3Ti4O12 type of compounds

We are reporting on the dielectric properties of NaCu3(Ti3Nb)O12 and Na(Cu2.5Ti0.5)Ti4O12 which are related to the perovskite CaCu3Ti4O12 compound. The NaCu3(Ti3Nb)O12 perovskite, that is doped with Nb cations at the Ti B-site, shows high values of dielectric constant. Using impedance spectroscopy (IS), we have observed that this compound is electrically inhomogeneous, showing a semiconductive bulk and insulating grain boundaries. The presence of a semiconductive bulk seems to indicate that a charge delocalization process is taking place through the Cu A′-site. In the case of the Na(Cu2.5Ti0.5)Ti4O12 perovskite, the dielectric response comes from the bulk material and the IS results show an intrinsic homogeneous material. In this compound, the large doping of Ti cations at the Cu A′-site prevents a charge delocalization process and the material becomes insulating. These results highlight the importance of the A′-sites in the charge delocalization process. In addition, Na(Cu2.5Ti0.5)Ti4O12 provides evidence of a soft-mode behavior that could be responsible for the relatively high intrinsic dielectric constant of CaCu3Ti4O12 and related compounds.