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

Abstract

A nominal Bi-excess starting composition of sodium bismuth titanate, Na_0.5Bi_0.51TiO_3.015 (NB_0.51T) produces dielectric ceramics that exhibit mixed n-type and oxide-ion conductivity with an ionic transport number of ∼0.1 at ≥600 °C. The bulk electrical conductivity, σ_b, of NB_0.51T ceramics under a dc bias field of ≤100 V cm⁻¹ has been investigated by impedance spectroscopy. Over the temperature range ∼550 to 750 °C, σ_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 °C in nitrogen using Pt electrodes. This is in contrast to that observed for other n-type perovskite titanates and oxygen-deficient rutile where σ_b is suppressed under a dc bias. This ‘unusual’ non-ohmic behaviour is attributed mainly to the influence of highly mobile oxygen vacancies in NB_0.51T. The field-enhanced σ_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 σ_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.