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

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 (PO2-0.015) and the optimized amount of Sr doping is x = 0.1 in Bi4-xSrxNbO8-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.