Thermodynamic properties of quaternary oxides in the system Ba–La–Fe–O by differential scanning calorimetry and solid-state electrochemical cells

Abstract

Two compounds, BaLa 2Fe 2O 7(s) and BaLaFeO 4(s) in the quaternary system Ba–La–Fe–O have been prepared by citrate-nitrate gel combustion route and characterized by X-ray diffraction analysis. Heat capacities of these two oxides were measured in the temperature range from 302 K to 832 K using a heat flux type differential scanning calorimeter. Two different types of solid-state electrochemical cells with CaF 2 as the solid electrolyte were employed to measure the e.m.f. as a function of temperature. The standard molar Gibbs energy of formation of the above oxides was calculated as a function of temperature from the e.m.f. data. The standard molar enthalpy of formation at 298.15 K, Δ f H m ∘ ( 298.15 K ) and the standard entropy S m ∘ ( 298.15 K ) , of these two oxides were calculated by second law method. The values of Δ f H m ∘ ( 298.15 K ) and S m ∘ ( 298.15 K ) obtained for BaLa 2Fe 2O 7(s) are: −3446.4 kJ · mol −1 and 246.6 J · K −1 · mol −1 whereas those for BaLaFeO 4(s) are: −2080.4 kJ · mol −1 and 95.0 J · K −1 · mol −1, respectively.