Profound Understanding of Effect of Transition Metal Dopant, Sintering Temperature, and pO₂ on the Electrical and Optical Properties of Proton Conducting BaCe_0.9Sm_0.1O_3-δ

This study reports the effect of transition metal (TM) substitution on the electrical and optical properties of BaCe_0.9Sm_0.1O_3-δ (BCS). Concentrations of 5-10 mol % of each of Fe and Co have been doped for the B-site of BCS by citric acid autocombustion method. Powder X-ray diffraction has revealed the formation of an orthorhombic perovskite-type structure. FTIR confirmed a distortion in the lattice upon TM-doping in BCS. Scanning electron microscopy (SEM) images of 1400 °C sintered samples have manifested a higher densification in BaCe_0.8Sm_0.1Co_0.1O_3-δ (BCSC10) with a grain size ∼11 μm compared to the parent compound BCS (∼2 μm). Thermogravimetric (TG) analysis showed a water uptake in case of BaCe_0.85Sm_0.1Co_0.05O_3-δ (BCSC5), while BaCe_0.85Sm_0.1Fe_0.05O_3-δ (BCSF5) did not show a noteworthy uptake of water. TG has also proved that the incorporation of Fe and Co in BCS did not improve the chemical stability in CO₂ at elevated temperature. The band gap estimated using Kubelka-Munk model based on the diffuse reflectance data was found to be the lowest for BCSC5 (2.47 eV). However, it increases upon lowering oxygen partial pressure (pO₂), which was interpreted by a band structure modifications. Among the samples investigated, BCSC10 sintered at 1400 °C showed the highest electrical conductivity of 0.02 S cm^-1 in air at 600 °C, while its proton mobility appears to be negligible under the investigated humidity atmosphere.