Phase relations in the Bi ₂ O ₃ –Mn ₂ O ₃ –M ₂ O ₃ (MFe, 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 Mn 3 ⁺ and Mn⁴⁺ oxidation states in mullite Bi 2 Mn 4 O 10 ‐based phases and Mn 4+ in sillenite Bi 12 MnO 20 ‐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‐Bi 2 O 3 and sillenite Bi₂₅MO₃₉ which have been largely incorrectly defined in the past. In contrast, in the binary system Bi 2 O 3 –Mn 2 O 3 the γ‐Bi 2 O 3 was not identified. The sillenite compounds Bi 12 MnO 20 and Bi 25 MO 39 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 BiGaO 3 and BiAlO 3 were not confirmed in the investigated systems. In the investigated systems, the mullite‐type Bi 2 Mn 4 O 10 and Bi 2 M 4 O 9 (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.