Phase Equilibrium in the System Ln–M–O: III. Ln=Gd at 1100°C

Abstract

Phase equilibrium in the system Gd–Mn–O has been established at 1100°C while varying the partial pressure of oxygen between 0 and 13.00 in −log (PO2/atm), and a phase diagram at 1100°C is presented as a Gd2O3–MnO–MnO2 system. Under the experimental conditions, Gd2O3, MnO, Mn3O4, GdMnO3, and GdMn2O5 phases are present at 1100°C, but Gd2MnO4, Mn2O3, and MnO2 are not stable in the system. The substantial difference from the previously studied La–Mn–O and Nd–Mn–O systems lies in the fact that the LnMn2O5-type phase is stable under the present experimental conditions. A wide range of nonstoichiometry has been found in the GdMnO3 phase coexisting with Gd2O3. X in GdMnO3+X ranges from −0.03 at log PO2=−9.47 to 0.05 at log PO2=0. Nonstoichiometry is represented by an equation, NO/NGdMnO3=3.00×10−4(log PO2)3+5.80×10−3(log PO2)2+3.52×10−2(log PO2)+0.0464, and the activities of components in solid solution are calculated from the equation. Similar to the case of LaMnO3, GdMnO3 seems to vary in composition between the Gd2O3-rich and Gd2O3-poor sides. Lattice constants of GdMnO3 produced under different oxygen partial pressures and those of GdMn2O5 prepared in air were determined, along with spacings and relative intensities of GdMn2O5. Standard Gibbs energies of reactions shown in the system were calculated and compared with previously reported values.