Thermodynamic studies on the systems Ln-Fe-O (Ln = Nd, Sm, Dy)

Abstract

Ternary oxides in the systems Ln–Fe–O (Ln = Nd, Sm, Dy) have been prepared by the citrate–nitrate route and characterised by X?ray diffraction analysis. The particle sizes of the ternary compounds were estimated to be in the range from 2 to 4 ?m. The enthalpy increments for five ternary oxides have been measured by a Calvet micro?calorimeter in the temperature range from 299 to 1000 K. Heat capacities of all these ternary oxides have been derived by differentiation of enthalpy increment values with respect to temperature. Heat capacity anomalies have been observed in all the five compounds; they are attributed to second?order magnetic order–disorder (antiferromagnetic to paramagnetic) transformation. The Néel temperatures (TN) evaluated in the present study for the perovskite?type compounds NdFeO?(s), SmFeO?(s) and DyFeO?(s) are 687, 673 and 648 K, respectively, whereas those for the garnet?type compounds Sm?Fe?O??(s) and Dy?Fe?O??(s) are 560 K each. Suitable solid?state electrochemical cells have been designed to measure the oxygen potential for the various equilibrium reactions involving the ternary oxides. The oxygen potential for the reaction: 2/3 Ln?O?(s) + 4/3 Fe(s) + O?(g) = 4/3 LnFeO?(s), where Ln = Nd, Sm and Dy, has been calculated from the measured EMFs of the cell: (–) Pt/{LnFeO?(s) + Ln?O?(s) + Fe(s)} // YDT/CSZ // {Fe(s) + Fe?.???(s)} / Pt(+). The values of ??(O?) obtained in this study were found to be in good agreement with the reported values when extrapolated to high temperatures. The oxygen potential for the equilibrium reaction: 18 LnFeO?(s) + 4 Fe?O?(s) + O?(g) = 6 Ln?Fe?O??(s), where Ln = Sm and Dy, has been calculated from the measured EMFs of the cell: (–) Pt/{Fe(s) + Fe?.???(s)} // CSZ // {Ln?Fe?O??(s) + LnFeO?(s) + Fe?O?(s)} / Pt(+). The values of ??(O?) obtained in this study were found to be significantly lower than the values reported in the literature. The standard molar Gibbs energy of formation of the ternary oxides in the systems Ln–Fe–O has been calculated from the results obtained in this study. A second?law analysis has been carried out to calculate the standard molar enthalpy of formation ?H°m(298.15 K) and the standard molar entropy S°m(298.15 K) of the ternary oxides NdFeO?(s), SmFeO?(s), DyFeO?(s), Sm?Fe?O??(s) and Dy?Fe?O??(s). Oxygen potential diagrams have been developed for the system Nd–Fe–O at 1350 K, and for the systems Sm–Fe–O and Dy–Fe–O at 1250 K. The isothermal oxygen potential diagram indicates that on reducing the oxygen partial pressure at 1350 K over the system Nd–Fe–O, the perovskite compound NdFeO?(s) dissociates to Fe(s) and Nd?O?(s). For the systems Sm–Fe–O and Dy–Fe–O, the diagrams show that on reducing the oxygen partial pressure at 1250 K, the garnet compounds Ln?Fe?O??(s) dissociate first to LnFeO?(s) and Fe?O?(s), followed by the dissociation of LnFeO?(s) to Ln?O?(s) and Fe(s). Two dimensional and three?dimensional chemical potential diagrams have been computed for the system Dy–Fe–O at 1250 K. The thermodynamic data obtained in the present study would be useful for predicting the stability of the ternary oxides in the systems Ln–Fe–O in different chemical environments at high temperatures and also for the computation of multicomponent phase diagrams involving these elements.