Phase relations in the Bi2O3–Mn2O3–M2O3 (MFe, Al, and Ga) pseudoternary systems

Abstract

This study establishes the subsolidus phase relations in the Bi₂O₃–Mn₂O₃–M₂O₃ (M = Fe, Al, and Ga) systems at 770°C in an oxidizing air environment, with a specific focus on identifying of phase stability and extension of solid solubility of new solid solutions. The pseudoternary nature of these systems is influenced by the presence of both Mn³⁺ and Mn⁴⁺ oxidation states in mullite Bi₂Mn₄O₁₀-based phases and Mn⁴⁺ in sillenite Bi₁₂MnO₂₀-based phases. Our findings reveal that the addition of M₂O₃ (where M = Fe, Al, and Ga) in small amounts (up to 1.5 mol%) to Bi₂O₃ promotes the formation of the γ-Bi₂O₃ phase. However, with increased M₂O₃ addition (up to 7 mol%), isomorphous sillenite compounds Bi₂₅MO₃₉ are formed. These findings clearly show differences between the two phases, γ-Bi₂O₃ and sillenite Bi₂₅MO₃₉ which have been largely incorrectly defined in the past. In contrast, in the binary system Bi₂O₃–Mn₂O₃ the γ-Bi₂O₃ was not identified. The sillenite compounds Bi₁₂MnO₂₀ and Bi₂₅MO₃₉ exhibit solid solubility in all three systems M = Fe, Al, and Ga over the entire composition range. Additionally, the perovskite phase BiFeO₃ exhibits an extended solid solubility, incorporating up to 32 at% of Mn as a substitution for Fe however the perovskite-type BiGaO₃ and BiAlO₃ were not confirmed in the investigated systems. In the investigated systems, the mullite-type Bi₂Mn₄O₁₀ and Bi₂M₄O₉ (M = Fe, Al, and Ga) form solid solutions with various compositional extensions, which depends on the difference of ionic size of M atoms (Fe, Al, and Ga) in comparison of Mn size. Based on experimental results, the three phase diagrams of the Bi₂O₃–Mn₂O₃–M₂O₃ (M = Fe, Al, and Ga) systems were constructed.