Colossal Permittivity and Dielectric Relaxations of (Nb, Al) Co-doped BaTiO3 Ceramics

Nb and Al co-doped BaTiO3 ceramics, BaTi0.98(Nb0.5Al0 5)(0.02)O-3, have a colossal permittivity (epsilon(r)approximate to 3 X 10(5), tan delta approximate to 0.2) at room-temperature. Three distinct relaxation processes are observed in the frequency range from 10(-1) to 10(7) Hz. The low-frequency relaxation between 10.1 and 10 Hz and the intermediate-frequency relaxation between 10(3) and 10(5) Hz are non-Debye relaxations, which are attributed to Maxwell-Wagner interfacial polarization, electrode interfacial polarization, and barrier layer capacitor. While the high-frequency process between 10(5) and 10(7) Hz is a typical Debye-type relaxation with an activation energy of E approximate to 15 meV and a frequency factor of f(0)approximate to 7 X 10(6) Hz. The small activation energy and relatively small frequency factor in the BaTi0.98(Nb0.5Al0.5)(0.02)O-3 ceramics indicate that this relaxation process may derive from local motion of electrons in the complex defect clusters, which results from co-doping and can be named as electron-pinned defect-dipoles. This study suggests that the electron-pinned defect-dipoles mechanism can be used to design colossal permittivity in perovskites, like BaTiO3.