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

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AbstractThis 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.