Microstructure and electrical conductivity of A-site fully stoichiometric Na0.5+xBi0.5−xTiO3−δ with different Na/Bi ratios

The effect of nominal Na/Bi ratio on the microstructure and electrical conductivity of A-site fully stoichiometric sodium bismuth titanates , Na 0.5+ x Bi 0.5− x TiO 3− δ , was investigated in this study. Bulk samples with x  = 0, 0.01, 0.03, 0.05, 0.07, and 0.1 were prepared by conventional solid state reaction method. The as-calcined powders primarily exhibited the perovskite structure, which was identified by X-ray diffraction. Electron microscopic investigation of the sintered samples, however, revealed the presence of secondary phases, the amounts of which were found to increase with increasing Na/Bi ratio. Further elemental analysis by energy dispersive spectroscopy indicated that the secondary phases were mainly composed of sodium titanates with different Na/Ti ratios. The grain bulk and grain boundary conductivities of Na 0.5+ x Bi 0.5− x TiO 3− δ , measured by two-probe AC electrochemical impedance spectroscopy , significantly increased with increasing Na/Bi ratio when x  ≤ 0.03, but remained almost constant at higher x . The synergetic effect of oxygen vacancy creation, grain size reduction, and secondary phase formation on the variation in the conductivity upon increasing the nominal Na/Bi ratio in Na 0.5+ x Bi 0.5− x TiO 3− δ was discussed.