Thermodynamics of BaNd2Fe2O7(s) and BaNdFeO4(s) in the system Basingle bondNdsingle bondFesingle bondO

Two compounds, BaNd2Fe2O7(s) and BaNdFeO4(s) in the quaternary system Basingle bondNdsingle bondFesingle bondO 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 CaF2(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, image (298.15 K) and the standard entropies, image (298.15 K) of these oxides were calculated by the second law method. The values of image (298.15 K) and image (298.15 K) obtained for BaNd2Fe2O7(s) are: −2756.9 kJ mol−1 and 234.0 J K−1 mol−1 whereas those for BaNdFeO4(s) are: −2061.5 kJ mol−1 and 91.6 J K−1 mol−1, respectively.