Electrical conductivity and conduction mechanisms in (Na_0.5Bi_0.5TiO₃)_1−x(BiScO₃)_x (0.00 ≤ x ≤ 0.25) solid solutions

The electrical properties of (Na_0.5Bi_0.5TiO₃)_1−x(BiScO₃)_x (NBT-BS, 0.00 ≤ x ≤ 0.25) solid solutions are established by ac impedance spectroscopy and electromotive force transport number measurements. The bulk conductivity decreases with increasing BS incorporation but the oxide-ion transport number remains high (>0.85) over a wide compositional range 0.00 ≤ x ≤ 0.15 and drops to ∼0.7 for x ≥ 0.20. NBT-BS solid solutions can only present either predominant oxide-ion conduction or mixed ionic-electronic conduction behaviour, indicating that oxide-ion conduction cannot be fully eliminated by incorporation of BS. This is in contrast from our previous study where incorporation of ∼7% BiAlO₃ (BA) can fully suppress the oxide-ion conduction in NBT. The conductivity–composition relationships of NBT-BS solid solutions are attributed to a competing effect from lattice expansion, which enlarges the channel for oxygen ion migration, with trapping between B-site acceptor ions, , and oxygen vacancies, , which decreases oxygen ion migration. Comparisons between NBT-BS, NBT-BA and NBT-BiGaO₃ (BG) solid solutions suggest that small acceptor ions on the B-site are more effective in trapping oxygen vacancies and consequently more effective to suppress the oxide-ion conduction and thus reduce dielectric loss at elevated temperatures.