Mixed ionic–electronic conduction and defect chemistry of (Na0.5Bi0.5TiO3)1−x(BiCoO3)x (0 ≤ x ≤ 0.06) solid solutions

Abstract

The electrical conductivity of Na_0.5Bi_0.5TiO₃ (NBT)-based materials has been investigated intensively in the past decade; however, the conduction behaviour with incorporation of variable-valence elements has been limited. Here (NBT)_1−x(BiCoO₃)_x (NBT-BC, 0 ≤ x ≤ 0.06) solid solutions were prepared by a solid-state reaction and their electrical properties were investigated by impedance spectroscopy. The results show that incorporation of BC changes the bulk conduction mechanism from predominant oxide-ion conduction for NBT to mixed ionic–electronic conduction for NBT-BC solid solutions. The n-type electronic component is induced by loss of oxygen at high temperatures which reduces Co³⁺ to Co²⁺, and the ionic conduction is suppressed due to trapping of oxygen vacancies by (or ). Bulk conductivity (σ_b) for single-phase compositions (0 ≤ x ≤ 0.04) shows a volcano-shape variation with increasing x due to a combined effect from the suppressed ionic contribution and the emergent electronic contribution. NBT-BC solid solutions represent a new Type II NBT with mixed conduction behavior but high σ_b and pave the way to understanding the electrical conduction mechanisms of NBT-based materials with the presence of variable-valence dopants, which may facilitate discovery of mixed conductors as electrode materials for solid oxide fuel cells.