Synthesis and characterization of carbon dioxide and boiling water stable proton conducting double perovskite-type metal oxides

In this paper, we report the synthesis, chemical stability and electrical properties of three new Ta-substituted double perovskite-type Ba₂Ca_2/3Nb_4/3O₆ (BCN). The powder X-ray diffraction (PXRD) confirms the formation of double perovskite-like structure Ba₂(Ca_0.75Nb_0.59Ta_0.66)O_6-δ, Ba₂(Ca_0.75Nb_0.66Ta_0.59)O_6-δ and Ba₂(Ca_0.79Nb_0.66Ta_0.55)O_6-δ. The PXRD of CO₂ treated (800 °C; 7 days) and water boiled (7 days) samples remain the same as the as-prepared samples, suggesting a long-term structural stability against the chemical reaction. The electrical conductivity of the investigated perovskites was found to vary in different atmospheres (air, dry N₂, wet N₂, H₂ and D₂O + N₂). The AC impedance investigations show bulk, grain-boundary and electrode contributions in the frequency range of 0.01 Hz to 7 MHz. Below 600 °C, the bulk conductivity in wet H₂ and wet N₂ was higher than in air, dry H₂ and dry N₂. However, an opposite trend was observed at high temperatures, which may be ascribed to p-type electronic conduction. The electrical conductivity of the investigated perovskites was decreased in D₂O + N₂ compared to that of H₂O + N₂ atmosphere. This clearly shows that the investigated Ta-doped BCN compounds exhibit proton conduction in wet atmosphere which was found to be consistent with water uptake. The water uptake was further confirmed by thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) characterization. Among the samples investigated, Ba₂(Ca_0.79Nb_0.66Ta_0.55)O_6-δ shows the highest proton conductivity of 4.8 × 10^-4 S cm^-1 (at 1 MHz) at 400 °C in wet (3% H₂O) N₂ or H₂, which is about an order of magnitude higher than the recently reported 1% Ca-doped LaNbO₄ at the same atmosphere and at 10 kHz. Crown