Heat capacities of some ternary oxides in the system Ba–Fe–O using differential scanning calorimetry

Ternary oxides such as BaFe12O19(s), BaFe2O4(s), Ba2Fe2O5(s), Ba3Fe2O6(s) and Ba5Fe2O8(s) of Ba–Fe–O system were synthesized by solid-state reaction route taking stoichiometric proportions of preheated BaCO3(s) and Fe2O3(s) powders of high purities. The standard molar heat capacities of these ternary oxides were determined using differential scanning calorimetry from 310 to 850 K. A heat capacity anomaly was observed for BaFe12O19(s), a member of magnetoplumbite group and largely used as hard ceramic magnet exhibited a heat capacity anomaly at 725 K in differential scanning calorimetric result. This anomaly is due to the magnetic order–disorder transition from ferrimagnetic state to paramagnetic state of the compound and the transition temperature has been termed as Curie temperature (TC). Thermodynamic functions such as ΔfHm° (298.15 K), Sm° (298.15 K)Sm° (298.15 K), Sm°(T)Sm°(T), Cp,m°(T)Cp,m°(T), Hm°(T)Hm°(T), {Hm°(T)−Hm° (298.15 K)}{Hm°(T)−Hm° (298.15 K)}, Gm°(T)Gm°(T), fef (free energy function), ΔfHm°(T) and ΔfGm°(T) for these ternary oxides were also calculated from 298 to 1000 K using the second law analysis.