Phase equilibrium in the system Ln-Mn-O VI:: Ln = Ho and Tb at 1100°C

Phase equilibria were established in Ho-Mn-O and Tb-Mn-O systems at 1100degreesC by varying the oxygen partial pressure from -log(P-O2/atm) = 0-13.00, and phase diagrams for the corresponding Ln(2)O(3)-MnO-MnO2 systems at 1100degreesC were presented. Stable Ln(2)O(3), MnO, Mn3O4, LnMnO(3), and LnMn(2)O(5) phases were found at 1100degreesC, whereas Ln(2)Mn(2)O(7), Ln(2)MnO(4), Mn2O3, and MnO2 were not found to be stable. Small nonstoichiometric ranges were found in the LnMnO(3) phase, with the composition of LnMnO(3) represented as functions of log(P-O2/atm), N-O/N-HoMnO3 = 9.0 x 10(-5)(log P-O2)(3) + 1.1 x 10(-3) (log P-O2)(2) + 6.0 x 10(-3)(log P-O2) + 4.7 x 10(-3) and NO/N-TbMnO3 = 2.00 x 10(-4) (log P-O2)3 + 3.40 x 10(-3) (log P-O2)(2) + 1.81 x 10(-2) log P-O2 + 3.47 x 10(-2). Activities of the components in the solid solution were calculated from these equations. The composition of LnMnO(3) may range from Ln(2)O(3) rich to Ln(2)O(3) poor, while MnO is slightly nonstoichiometric, being oxygen rich and LnMn(2)O(5) seems to be nonstoichiometric. Lattice constants of LnMnO(3) quenched at different oxygen partial pressures and of LnMn(2)O(5) quenched in air were determined. The standard Gibbs energy changes of the reactions appearing in the phase diagrams were also calculated. The relationship between the tolerance factor of LnMnO(3) and DeltaG(0) of reaction, (1/2)Ln(2)O(3) + MnO + (1/4)O-2 = LnMnO(3), is shown graphically. (C) 2003 Elsevier Inc. All rights reserved.