Evidence of the ultrahigh dielectric constant of CaSiO3-doped CaCu3Ti4O12 from its dielectric response, impedance spectroscopy, and microstructure

The temperature and frequency dependences of the dielectric responses and the complex impedance spectra of polycrystalline CaCu3Ti4O12 (CCTO) doped with CaSiO3 have been investigated. The dielectric responses show two Maxwell-Wagner-type relaxations including insulating layers of grain boundaries and domain boundaries and semiconducting grains. The behavior of the temperature dependence of the dielectric constant at 1 kHz is demonstrated to be determined by the frequency dependence of the dielectric constant of two superposed dielectric relaxations at different temperatures. In contrast to the undoped CCTO, an extra semicircle clearly appears in the complex impedance plots of CaSiO3-doped CCTO. The results further confirm the presence of the domain boundaries, and their resistivity is lower than that of the grain boundaries. The microstructures have been characterized and the compositions of the grains, grain boundaries, and the second phases have been determined quantitatively. The Si ions are proved to be segregated to the grain boundaries. The grain-boundary phases consist of a main Si-rich amorphous phase and a small amount of nanosized Cu-rich particles. The Cu-rich particles are present at grain edges or corners of the microstructures. The variations of the electrical conduction and the dielectric responses are related to the reduction of the concentration of the excess Cu ions along the grain boundaries.