Enhanced conductivity and nonlinear voltage–current characteristics of nonstoichiometric BaTiO3 ceramics

The electrical conductivity of both BaO-deficient and TiO2-deficient BaTiO3 ceramics shows nonohmic, low-field characteristics at temperatures >∼200°C in contrast to stoichiometric BaTiO3 for which the electrical conductivity is independent of applied voltage. The nonlinearity is observed in both bulk and grain-boundary resistances of ceramics that are both porous (∼82%) and nonporous (∼98%) and is not associated with interfacial phenomena such as Schottky barriers and memristors or with charge injection from the electrodes. Results, shown as a function of time over the temperature range 200°–750°C with field gradients in the range ∼0.5–20 V/mm, indicate that an excited state is reached that is time, temperature, and field dependent. This effect appears to be caused by departures from local electroneutrality in the defect structure of nonstoichiometric BaTiO3 which are reduced by electron transfer on application of a dc bias, leading to a more conducting, low-level excited state in which holes associated with underbonded oxygens, presumably as O− ions, are the principal charge carriers. Ceramics gradually return to their ground state in two stages on removal of the dc bias and the conductivity decreases overall by two to three orders of magnitude