Thermodynamic studies on SrFe 12O 19(s), SrFe 2O 4(s), Sr 2Fe 2O 5(s) and Sr 3Fe 2O 6(s)

Abstract

The citrate–nitrate gel combustion route was used to prepare SrFe 2O 4(s), Sr 2Fe 2O 5(s) and Sr 3Fe 2O 6(s) powders and the compounds were characterized by X-ray diffraction analysis. Different solid-state electrochemical cells were used for the measurement of emf as a function of temperature from 970 to 1151 K. The standard molar Gibbs energies of formation of these ternary oxides were calculated as a function of temperature from the emf data and are represented as Δ f G m ∘ (SrFe 2O 4, s, T)/kJ mol −1 (±1.7)=−1494.8+0.3754 ( T/K) (970⩽ T/K⩽1151). Δ f G m ∘ (Sr 2Fe 2O 5, s, T)/kJ mol −1 (±3.0)=−2119.3+0.4461 ( T/K) (970⩽ T/K⩽1149). Δ f G m ∘ (Sr 3Fe 2O 6, s, T)/kJ mol −1 (±7.3)=−2719.8+0.4974 ( T/K) (969⩽ T/K⩽1150). Standard molar heat capacities of these ternary oxides were determined from 310 to 820 K using a heat flux type differential scanning calorimeter (DSC). Based on second law analysis and using the thermodynamic database FactSage software, thermodynamic functions such as Δ f H°(298.15 K), S°(298.15 K) S°( T), C p °( T), H°( T), { H°( T)– H°(298.15 K)}, G°( T), free energy function ( fef), Δ f H°( T) and Δ f G°( T) for these ternary oxides were also calculated from 298 to 1000 K.