Thermal dissociation of RMnO3 (R=Dy, Yb, Lu)

The phase equilibria involved in the thermal dissociation of RMnO3 (R = Dy, Yb, Lu) were Studied in the range 973-1173 K by a static method in a vacuum circulation unit and by x-ray diffraction analysis of quenched solid phases. The RMnO3 manganites were shown to dissociate by the reaction RMnO3 = 1/2R(2)O(3) + MnO + 1/4O(2). The temperature dependences of the equilibrium oxygen pressure and Gibbs energy change in this reaction were determined for the three compounds. The experimental data were used to evaluate the standard thermodynamic functions of formation of RMnO3 from R2O3 and Mn2O3: Delta H-0(T) = -88.93 kJ/mol, Delta S-0(T) 46.56 J/(mol K) for DyMnO3; Delta H-0(T) = -130.95 kJ/mol, Delta S-0(T) = 86.25 J/(mol K) for YbMnO3; Delta H-0(T) -142.94 kJ/mol, Delta S-0(T) = 102.87 J/(mol K) for LuMnO3.