Effect of substitution of B-sites by Mn, Fe and Co in double perovskite-type Ba₃CaNb₂O₉ on structure and electrical properties

A novel family of metal oxides of chemical formula Ba₂Ca _(1-x-y)M_(x+z)Nb_(1-z+y)O_6-δ (M = Mn, Fe and Co) was developed as mixed ion and electronic conductors (MIECs) for application in solid oxide fuel cells (SOFCs). Investigated samples were synthesized by a ceramic method at 1400 °C in air and characterized using powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), dilatometry, chemical titration and electrochemical ac impedance spectroscopy (EIS). The Rietveld analysis of PXRD and SAED support the formation of ordered double perovskite-type structure with space group Fm3m and chemical stability in 5000 ppm H₂S/H₂ at 600 °C. Iodometric titration shows the valences of Mn and Co were 2+ and 3+, while Fe shows 3+ and 4+ states. SEM reveals that the porosity of the samples is highly affected by dopants. The thermal expansion coefficients (TEC) of the as-prepared samples are very comparable to the oxide conducting Y-doped ZrO₂. Ba₂Ca_0.79Co_0.5Nb _0.71O_6-δ exhibits the total conductivity of 3.7 × 10^-2 S cm^-1 at 816 °C in air. The trend in the conductivity of Ba₂Ca_(1-x-y)M_(x+z)Nb _(1-z+y)O_6-δ could be correlated to the reduction of the dopants and the subsequent loss of oxide ions at elevated temperatures.