Non-ohmic conduction in sodium bismuth titanate: the influence of oxide-ion conduction

A nominal Bi-excess starting composition of sodium bismuth titanate, Na0.5Bi0.51TiO3.015(NB0.51T) produces dielectric ceramics that exhibit mixed n-type and oxide-ion conductivity with an ionic transport number of similar to 0.1 at >= 600 degrees C. The bulk electrical conductivity,sigma(b), of NB0.51T ceramics under a dc bias field of <= 100 V cm(-1)has been investigated by impedance spectroscopy. Over the temperature range similar to 550 to 750 degrees C,sigma(b)increases by up to one order of magnitude under the dc bias and returns to its initial value on removal of the bias. The enhancement of conductivity is dependent on temperature, atmosphere, dc bias field and the electrode materials. A maximum conductivity enhancement of >2000% is achieved at 600 degrees C in nitrogen using Pt electrodes. This is in contrast to that observed for other n-type perovskite titanates and oxygen-deficient rutile where sigma(b)is suppressed under a dc bias. This 'unusual' non-ohmic behaviour is attributed mainly to the influence of highly mobile oxygen vacancies in NB0.51T. The field-enhanced sigma(b)is best described to be a consequence of increased pumping of oxygen from the cathode to the anode, in which the electrode reactions play an important role. In addition, dissociation of defect clusters may also contribute to the enhanced sigma(b)under a dc bias. The high, fast and reversible response to the dc bias voltage may expand the potential application of NBT-based materials to memory devices.