Phase relations in the Bi$_2$O$_3$–Mn$_2$O$_3$–M$_2$O$_3$ (M=Fe, Al, and Ga) pseudoternary systems

This study establishes the subsolidus phase relations in the Bi$_2$O$_3$–Mn$_2$O$_3$–M$_2$O$_3$ (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 Mn$^{4+}$ 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 M$_2$O$_3$ (where M = Fe, Al, and Ga) in small amounts (up to 1.5 mol%) to Bi$_2$O$_3$ promotes the formation of the γ-Bi$_2$O$_3$ phase. However, with increased M$_2$O$_3$ addition (up to 7 mol%), isomorphous sillenite compounds Bi$_{25}$MO$_{39}$ are formed. These findings clearly show differences between the two phases, γ-Bi$_2$O$_3$ and sillenite Bi$_{25}$MO$_{39}$ 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 BiFeO$_3$ 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 Bi$_2$O$_3$–Mn$_2$O$_3$–M$_2$O$_3$ (M = Fe, Al, and Ga) systems were constructed.