Thermodynamic studies on SrFe12O19(s), SrFe2O4(s), Sr2Fe2O5(s) and Sr3Fe2O6(s)

摘要

The citrate-nitrate gel combustion route was used to prepare SrFe2O4(s), Sr2Fe2O5(s) and Sr3Fe2O6(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 Delta(f) G(m)degrees (SrFe2O4, s, T)/kJ mol(-1) (+/- 1.7)=-1494.8+0.3754(T/K) (970 <= T/K <= 1151). Delta(f) G(m)degrees (Sr2Fe2O5, s, T)/kJ mol(-1) (+/- 3.0)=-2119.3+0.4461 (T/K) (970 <= T/K <= 1149). Delta(f) G(m)degrees (Sr3Fe2O6, 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 3 10 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 Delta H-f degrees(298.15K), S degrees(298.15K) S degrees(T), C-p degrees(7), H degrees(T), {H degrees(T)-H degrees(298.15K)}, G degrees(T), free energy function (fef), Delta H-f degrees(T) and Delta(f)G degrees(T) for these ternary oxides were also calculated from 298 to 1000 K. (c) 2006 Elsevier Inc. All rights reserved.