Re-entrant relaxor ferroelectric behaviour in Nb-doped BiFeO3-BaTiO3 ceramics

BiFeO3-BaTiO3 (BF-BT) solid solutions exhibit great promise as the basis for high temperature piezoelectric transducers and energy storage dielectrics, but the fundamental mechanisms governing their functional properties require further clarification. In the present study, both pure and niobium-doped 0.7BF-0.3BT ceramics are synthesized by solid state reaction and their structure-property relationships are systematically investigated. It is shown that substituting a low concentration of Ti with Nb at a level of 0.5 at% increases the resistivity of BF-BT ceramics and facilitates ferroelectric switching at high electric field levels. Stable planar piezoelectric coupling factor values are achieved with a variation from 0.35 to 0.45 over the temperature range from 100 to 430 degrees C. In addition to the ferroelectric-paraelectric phase transformation at the Curie point (similar to 430 degrees C), a frequency-dependent relaxation of the dielectric permittivity and associated loss peak are observed over the temperature range from -50 to +150 degrees C. These effects are correlated with anomalous enhancement of the remanent polarization and structural (rhombohedral) distortion with increasing temperature, indicating the occurrence of a re-entrant relaxor ferroelectric transformation on cooling. The results of the study provide new insight into the thermal evolution of structure and the corresponding functional properties in BF-BT and related solid solutions.