Sillén–Aurivillius phase bismuth niobium oxychloride, Bi4NbO8Cl, as a new oxide-ion conductor

Abstract

Sillén–Aurivillius phase Bi₄NbO₈Cl consists of Bi₂O₂⁺/NbO₃⁻/Bi₂O₂⁺/Cl⁻ layers and oxide-ion conduction is expected to occur in its Bi₂O₂⁺ layer. Here, we report the influence of partial substitutions of Bi with Ca²⁺, Sr²⁺, Ba²⁺, La³⁺, Ga³⁺ and Sn⁴⁺ on the electrical properties. It was found that substitution with low valence cations is effective for increasing electrical conductivity, and in particular, Sr²⁺ substitution is the most effective for this purpose. The dependence of electrical conductivity on the oxygen partial pressure of a Sr-doped sample is quite low (P_O2^−0.015) and the optimized amount of Sr doping is x = 0.1 in Bi_4−xSr_xNbO_8−δCl. The electromotive force of N₂/O₂ gas concentration cell is 90% of the theoretical value and the tracer diffusion constant (D) estimated by ¹⁸O₂ diffusion almost corresponds with that estimated from conductivity. Considering the activation energy of the D value, oxide-ion conductivity in Sillén–Aurivillius phase Bi₄NbO₈Cl mainly occurs along the Bi₂O₂²⁺ layer, which indicates that Bi₄NbO₈Cl is a new family of fast oxide-ion conductors.