The dielectric permittivity of Bi7−xLaxTi4NbO21 (x=0–1.75) ceramics at ambient temperature is stable in a wide range of frequencies and can be modulated from 150 to 320 by compositional modification. With increasing La3+ content, both the phase transition temperatures downshift and the phase transition peaks become diffusive when x⩾1.00. The P-E hysteresis loops indicated that the remanent polarization 2Pr of Bi6.5La0.5Ti4NbO21 has a maximum of 24.4μC∕cm2. The Raman scattering experiments revealed that the Bi3+ in [Bi2O2]2+ layers is substituted by La3+ when x⩾1.00 and there is no Bi3+ at the A sites being substituted further when x⩾1.50.

The ac data in terms of impedance and dielectric loss (tanδ) were exploited simultaneously to probe the dielectric relaxation mechanisms in Bi5TiNbWO15 ceramics. It was found that two distinct relaxation mechanisms exist in the low-frequency range (10Hz–5MHz). One is attributed to the grain boundary relaxation and the other is associated with oxygen ion diffusion. Furthermore, the temperature dependence of the oxygen vacancy relaxation strength is analogous to the Curie-Weiss law and follows the traditional point defect relaxation theory. These results could be helpful to understand the phenomena related to ferroelectric fatigue, oxygen ion conductivity, etc.