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, O-2(9)/ {CaO(s) + CaF2(s)}//CaF2(s)//{SrFe12O19(s) + SrF2(s) + Fe2O3(s)}/O-2(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: Delta(f)G(m)(o) (SrFe12O19, , s, T)/kJ mol(-1) (+/-1.3) = -5453.5 + 1.5267 x (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, Delta(f)H(m)(o)(298.15 K) and the standard molar entropy, S-m(o) (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. (C) 2004 Elsevier Ltd. All rights reserved.