Donor-doping and reduced leakage current in Nb-doped Na_(0.5)Bi_(0.5)TiO_3

摘要

Low levels of so-called "donor-doping" in titanate-based perovskite oxides such as La for Ba, Sr, and Nb for Ti in (Ba, Sr)TiO_3 can significantly reduce the resistivity of these typical (d~0) dielectric materials and expand application areas to positive temperature coefficient resistors, thermoelec-trics, conductive wafers as thin film substrates, and solid oxide fuel cell anode materials. Here, we show low levels of Nb-doping (≤ 1 at. %) on the Ti-site in the well-known lead-free piezoelectric perovskite oxide Na_(0.5)Bi_(0.5)TiO_3 (NBT) produces completely different behaviours whereby much higher resistivity is obtained, therefore indicating a different donor-doping (substitution) mechanism. There is a switch in conduction mechanism from oxygen-ions in undoped NBT with an activation energy (E_a) of ＜0.9 eV to electronic (band gap) conduction in 0.5-1 at. % Nb-doped NBT with E_a～ 1.5-1.8 eV. This demonstrates the necessity of further systematic doping studies to elucidate the defect chemistry of NBT which is clearly different to that of (Ba,Sr)TiO_3. This defect chemistry needs to be understood if NBT-based materials are going to be manufactured on a large scale for commercial applications. This study also illustrates different donor-doping mechanisms to exist within the family of d~0 titanate-based perovskites.