Dielectric relaxation at high temperatures induced by oxygen vacancies at grain boundary in Na-doped barium strontium titanium ceramics

The temperature and frequency dependence of dielectric properties of A-site lower valence cation Na-doped Barium strontium titanate (Ba_1-xNa_x)_0.9Sr_0.1TiO_{3 -} (x = 0.01, 0.03, 0.05, 0.08, 0.1, 0.15, 0.2 and 0.3) ceramics have been investigated up to 450 C from 100 Hz to 1 MHz. Other than dielectric properties at low temperature, the relaxation behavior above 150 C for all the compositions was observed with broad dielectric peaks (T), tan (T) and (f). The universal dielectric response associated with the activation energy of 0.79-0.98 eV and complex impedance properties indicate that with the increasing of Na, the concentration of oxygen vacancies inhabiting grain boundary increases and under thermal stimulation the tunneling effect of oxygen vacancies strengthens, which dominate for the relaxation at high temperatures.