Oxide ion conductivity in doped bismuth gallate mullite type oxide, Bi₂Ga₄O₉

Mullite-phase bismuth gallate (Bi₂Ga₄O₉) was successfully synthesized with partial substitution of bismuth with alkaline-earth cation, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. The effect of this substitution on the electrical conductivity was investigated. In this study, substitution with Ca²⁺ of Bi site was further studied for increasing the ionic conductivity as well as the phase stability in reducing atmospheres. Substitution with Ca²⁺ was found to be the most effective and with 12.5 mol% of Ca²⁺ as the optimized doping amount. Conductivity and stability in reducing atmospheres was increased down to p_O2 ≤ 10⁻¹⁹ atm while keeping the conductivity of σ = 2.6 × 10⁻² S·cm⁻¹ at 973 K largely independent of oxygen partial pressure. Oxygen permeation analysis estimates 76% of theoretical oxygen permeation rate at 973 K suggesting main charge carrier is oxide ion. Partial electronic conductivity was measured with the ion blocking method. Oxide ion conductivity is dominated over wide p_O2 range excepting for hole conduction at high p_O2. Density functional theory (DFT) analysis on oxide ion diffusion route suggests oxygen hoping through lattice vacancy is main pathway for oxide ion conductivity in this doped Bi₂Ga₄O₉. Despite the low oxide ion conductivity in Mullite-phase oxide, it was found that Ca doped Bi₂Ga₄O₉ shows good oxide ion conductivity over wide p_O2 range.