Thermochemical studies on SrFe12O19(s)

The citrate–nitrate gel combustion route was used to prepare SrFe12O19(s) powder sample and the compound was characterized by X-ray diffraction analysis. A solid-state electrochemical cell of the type: (−)Pt, O2(g)/{CaO(s) + CaF2(s)}//CaF2(s)//{SrFe12O19(s) + SrF2(s) + Fe2O3(s)}/O2(g), Pt(+) was used for the measurement of emf as a function of temperature from 984 to 1151 K. The standard molar Gibbs energy of formation of SrFe12O19(s) was calculated as a function of temperature from the emf data and is given by: Δ f G m ∘ (SrFe12O19, s, T)/kJ mol−1 (±1.3) = −5453.5 + 1.5267 × (T/K). Standard molar heat capacity of SrFe12O19(s) was determined in two different temperature ranges 130–325 K and 310–820 K using a heat flux type differential scanning calorimeter (DSC). A heat capacity anomaly was observed at 732 K, which has been attributed to the magnetic order–disorder transition from ferrimagnetic state to paramagnetic state. The standard molar enthalpy of formation, Δ f H m ∘ (298.15 K) and the standard molar entropy, S m ∘ (298.15 K) of SrFe12O19(s) were calculated by second law method and the values are −5545.2 kJ mol−1 and 633.1 J K−1 mol−1, respectively.