Thermodynamics of BaNd 2Fe 2O 7(s) and BaNdFeO 4(s) in the system Ba [sbnd]Nd [sbnd]Fe [sbnd

Abstract

Two compounds, BaNd 2Fe 2O 7(s) and BaNdFeO 4(s) in the quaternary system Ba Nd Fe O were prepared by citrate–nitrate gel combustion route and characterized by X-ray diffraction analysis. Heat capacities of these two oxides were measured in two different temperature ranges: (i) 130–325 K and (ii) 310–845 K, using a heat flux type differential scanning calorimeter. Two different types of solid-state electrochemical cells with CaF 2(s) as the solid electrolyte were employed to measure the e.m.f. as a function of temperature. The standard molar Gibbs energies of formation of these quaternary oxides were calculated as a function of temperature from the e.m.f. data. The standard molar enthalpies of formation from elements at 298.15 K, Δ f H m ° (298.15 K) and the standard entropies, S m ° (298.15 K) of these oxides were calculated by the second law method. The values of Δ f H m ° (298.15 K) and S m ° (298.15 K) obtained for BaNd 2Fe 2O 7(s) are: −2756.9 kJ mol −1 and 234.0 J K −1 mol −1 whereas those for BaNdFeO 4(s) are: −2061.5 kJ mol −1 and 91.6 J K −1 mol −1, respectively.