Effect of La3+ and Eu3+ doping on the structural, microstructural, and ferroelectric properties of bismuth ferrite ceramics

This work investigates a series of ten ceramic samples with the compositions Bi_1-xLa_xFeO₃ (x = 0.05, 0.10, 0.15), Bi_1-yEu_yFeO₃ (y = 0.05, 0.10, 0.15), and Bi_1-(x + y)La_xEu_yFeO₃ (x = y = 0, 0.025, 0.05, 0.075). The samples were synthesized through a hydro-evaporation method and subsequently subjected to sintering at 835 °C. The focus was on the relationship between the samples' structural and microstructural properties and their ferroelectric behavior. The study also emphasized the distinct effects of the La³⁺ and Eu³⁺ dopant ions on the material properties, contributing to a deeper understanding of the mechanisms influencing ferroelectricity in these complex ceramic systems. XRD analysis revealed that all lanthanum and europium substituted bismuth ferrite ceramics had the rhombohedral (R3c) structure as the dominant one, while the samples doped with 15 mol% of dopant ions contained the orthorhombic phases as well. The partial substitution of Bi³⁺ ions with 10 mol% of La³⁺ or Eu³⁺ and 15 mol% of La³⁺ resulted in significantly enhanced ferroelectric responses. These compositions showed larger remnant polarizations, higher coercive fields, and nearly square-shaped hysteresis curves compared to the undoped and other doped samples. The Bi_0.85La_0.15FeO₃ sample demonstrated the highest remnant electric polarization (P_r = 24 μC/cm²) when subjected to a 160 kV/cm electric field. Due to its unique structural and microstructural properties, the material exhibited nearly saturated hysteresis loops under high electric fields, accompanied by minimal leakage currents. The Bi_0.85Eu_0.15FeO₃ and Bi_0.85La_0.075Eu_0.075FeO₃ samples exhibited poor ferroelectric responses due to non-polar, orthorhombic structures in their composition. Generally, La and Eu-doping significantly reduced the leakage current densities of bismuth ferrite owing to a decrease in the concentration of oxygen vacancies, which are known to be a primary cause of electric conductivity in undoped ceramics.