THERMOCHEMICAL STABILITY OF BAFE12O19 AND BAFE2O4 AND PHASE-RELATIONS IN THE BA-FE-O TERNARY-SYSTEM

The stability of BaFe12O19 and BaFe2O4 was studied by the oxygen coulometric titration technique between 700-degrees-C and 1000-degrees-C using a solid-state electrochemical cell. This temperature range is technologically important for the deposition of BaFe2O19 magnetic 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 BaFe12O19 composition was constructed in this temperature range. The standard Gibbs free energy change for the decomposition of BaFe12O19 into BaFe2O4, Fe3O4, and O2 is given by the expression DELTAG-degrees[J/mol] = 7.23 X 10(5) - 480T. In the oxygen pressure-temperature domain, the thermodynamic stability limits of BaFe12O19 and BaFe2O4 are given by the expressions ln[P(O2)(atm)] = 69.37 - 1.04 X 10(5) T-1 and ln[P(O2)(atm)] = 27.68 - 7.12 X 10(4) T-1, respectively. The stability limits determined here help define the process conditions for the successful synthesis of these phases.