Crystal structure and proton conductivity of complex perovskite Ca5-yBayNb2-xZrxTiO12 ceramics

In recent years, the considerable attention has been paid to the characterization of perovskite-structured electrolytes because of their proton conductivities for the application to the intermediate temperature fuel cells... [ view full abstract ]

In recent years, the considerable attention has been paid to the characterization of perovskite-structured electrolytes because of their proton conductivities for the application to the intermediate temperature fuel cells which can operate at the lower temperatures below 600°C. In such the electrolytes, the acceptor doping has been performed to introduce the oxygen vacancy which takes place the proton incorporation in the lattice; the Y-doped BaZrO₃ and BaCeO₃ perovskite compounds exhibit the high proton conductivity (10^-2 S/cm at 600 °C). On the other hand, the compounds with a perovskite-related structure such as the brownmillerite, double perovskite, and complex perovskite have been also considered to be a candidate as a proton conducting electrolytes. Thus, the influences of Zr substitution for Nb and Ba substitution for Ca on electronic conductivity of complex perovskite-structured Ca₅Nb₂TiO₁₂ ceramics were investigated in this study. The Ca_5-yBa_yNb_2-xZr_xTiO₁₂ ceramics were synthesized using the conventional solid-state reaction method, and sintered at 1500-1560 °C for 5 h in air. The XRPD profiles of the Ca₅Nb_2-xZr_xTiO₁₂ ceramics showed the single phase, which is an orthorhombic structure (Pnma), in the composition range of x = 0 - 0.12, suggesting the introduction of oxygen vacancy in the lattice. In this composition range, the electrical conductivity of the ceramics was improved by the Zr substitution for Nb because of an introduction of oxygen vacancy, and the proton transport number of the ceramics was 97 % in the temperature range of 400-800 °C. On the other hand, the crystal structure of the Ca_5-yBa_yNb_1.88Zr_0.12TiO₁₂ ceramics changed from orthorhombic to cubic structure as the y varied from 0 to 4. As a result, the highest electrical conductivity of 1.5×10^-3 S/cm was obtained for CaBa₄Nb_1.88Zr_0.12TiO₁₂ (x = 0.12 and y = 4) ceramic with cubic structure at 800^oC, suggesting the protonic conduction.