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.