Effects of Bi deficiency on the microstructural and conductive properties of Na0.5Bi0.5TiO3 (NBT) perovskites

The effect of Bi deficiency on the microstructure and the conductive properties of sodium bismuth titanates (NBTs) have been investigated experimentally. A series of NBTs samples with different Bi deficiencies ranging from 0 to 10 at% were prepared with conventional solid state reaction. X-ray diffraction (XRD), backscattering electron images (BSE), and energy dispersive spectroscopy (EDS) were applied to investigate the phase structure, phase stability, and phase compositions. The total electrical conductivity of NBTs was measured by 2-probe AC impedance as functions of Bi deficiency, temperature, and oxygen partial pressure. The contribution of sodium ion was discussed. The proton and oxygen ion transfer numbers were measured by the EMF method and a combined method of OCV and EIS, respectively. As-calcined powders were found primarily in perovskite structure (R3c space group), while increasing amount of secondary phases with Bi deficiency were observed in the sintered pellets. It indicates the thermal history significantly affects the phase stability of NBTs. The electrical conductivity in NBTs was found PO2 independent and significant enhancement could be achieved in Bi-deficient NBTs. The influences of Mg doping on the electrical conductivity in the Bi-deficient NBTs were also examined and found different effects depending on the Bi deficiency. Discussions and experimental results on the contributions of different charge carriers in NBTs suggested the oxygen ion be the predominant contributor to the total electrical conductivity, which is in agreement with the literature findings.