Phase Equilibrium in the System Ln-Mn-O: I. Ln=La at 1100°C

Phase equilibrium was established in the system La-Mn-O at 1100°C by changing the oxygen partial pressure from 0 to 15.00 in-log (PO2/atm). A phase diagram at 1100°C in presented for the system La2O3–MnO–MnO2. Under the experimental conditions, La2O3, MnO, Mn3O4, and LaMnO3 phases are present at 1100°C. La2MnO4, Mn2O3, and MnO2 are not stable. Wide ranges of nonstoichiometry were found is the LaMnO3 phase. x ranges from -0.050 at log PO2=-13.05 to 0.18 at logPO2 =0 in the form of LaMnO3+x. The nonstoichiometry is represented with the equation, NO/NLaMnO3=1.94×10-4(logPO2)3+ 5.24×10-3 (logPO2)2+5.24×10-2(logPO2)+0.179, and activities of the components in the solid solutions are calculated using this equation. LaMnO3 seems to have a range of compositions from La2O3-rich to poor. Also MnO has a small nonstoichiometry at the oxygen-rich side. Lattice constants of LaMnO3 were determined at different oxygen partial pressures. The crystal form of LaMnO3 seems to change from orthorhombic to hexagonal as the oxygen partial pressure becomes higher. The standard Gibbs energy changes of the reactions in the phase diagram presented were calculated.