Thermochemical stability of BaFe₁₂O₁₉and BaFe₂O₄and phase relations in the Ba-Fe-O ternary system

The stability of BaFe₁₂O₁₉and BaFe₂O₄was studied by the oxygen coulometric titration technique between 700 °C and 1000 °C using a solid-state electrochemical cell. This temperature range is technologically important for the deposition of BaFe₂O₁₉magnetic thin films. The thermodynamic information obtained from the titration measurements was corroborated with structural identification of phases prepared under electrochemically controlled conditions. Accordingly, a section of the Ba–Fe–O ternary phase diagram around the BaFe₁₂O₁₉composition was constructed in this temperature range. The standard Gibbs free energy change for the decomposition of BaFe₁₂O₁₉into BaFe₂O₄, Fe₃O₄, and O₂is given by the expression ΔG°[J/mol] = 7.23 × 10⁵−480T. In the oxygen pressure-temperature domain, the thermodynamic stability limits of BaFe₁₂O₁₉and BaFe₂O₄are given by the expressions In[Po₂(atm)] = 69.37 −1.04 × 10⁵T⁻¹and In[Po₂(atm)] = 27.68 −7.12 × 10⁴T⁻¹, respectively. The stability limits determined here help define the process conditions for the successful synthesis of these phases.