Colossal permittivity and dielectric relaxations of (Nb, Al) co-doped BaTiO3 ceramics

Abstract

Nb and Al co-doped BaTiO3 ceramics, BaTi0.98(Nb0.5Al0.5)0.02O3, have a colossal permittivity (εr≈3×105, tanδ≈0.2) at room-temperature. Three distinct relaxation processes are observed in the frequency range from 10-1 to 107 Hz. The low-frequency relaxation between 10-1 and 10 Hz and the intermediate-frequency relaxation between 103 and 105 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 105 and 107 Hz is a typical Debye-type relaxation with an activation energy of E≈15 meV and a frequency factor of f0≈7×106 Hz. The small activation energy and relatively small frequency factor in the BaTi0.98(Nb0.5Al0.5)0.02O3 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.