Sol–gel synthesis, morphological analysis, characterization, and oxygen ion transport properties of binary doped (BiO₁.₅)1–x–y(HoO₁.₅)x(ErO₁.₅)y system

Bi₂O₃, with an anion–defective sublattice, exhibits superior ion conductivity, and therefore it appears to be an electrolyte candidate for intermediate–temperature SOFC applications. Literature reports that binary dopant–doped Bi₂O₃ systems offer higher ionic conductivity compared to single dopant ones. In this study, compositions of Er:Ho co–doped Bi₂O₃ were produced using the sol–gel synthesis technique. The investigation focused on how the dopant concentration and content ratios affect their thermoelectrical and microstructural properties. The XRD patterns revealed that all compositions were stable with the cubic δ–phase, but a peak shift depending on doping was observed, implying lattice contraction. The DTA plot of the composition (BiO₁.₅)₀.₉₀(HoO₁.₅)₀.₀₅(ErO₁.₅)₀.₀₅ clearly displayed an endothermal peak appearing at 600 °C, suggesting the order–disorder transition. The highest conductivity was found to be 0.27 S/cm at 700 °C for the same composition. As the percentage of dopants increased, the conductivity decreased, indicating a decay in the polarization power on the anion sublattice. The FE–SEM images revealed particle contraction due to increased doping, which means an increase in particle boundary resistance leading to a decrease in conductivity. The Raman shift was observed depending on doping, implying that partial cation substitution was successful. The AFM diagrams revealed the presence of island–like structures indicative of atomic clusters. The EDAX pattern showed that Bi and O elements exhibited a homogeneous distribution, while Ho and Er were localized.