Phase Stability of the System Mg–Fe–O

Abstract

The electrical conductivity and thermoelectric power of the system Mg–Fe–O were measured as functions of oxygen activity (aO2) and cationic composition in the temperature range 1100≤T/°C≤1300. The isotherms of both electrical properties underwent a conspicuous change in trend at the same, characteristic oxygen activities, which was ascribed to aO2-induced phase transitions from rock-salt structure to spinel structure to corrundum structure with successively increasing aO2's. The region of thermodynamic stability of each phase could be determined from either of the isotherms to construct phase diagrams of the first kind (i.e., log aO2 vs 1/T) for fixed compositions and the second kind (i.e., log aO2 vs composition) at fixed temperatures. From the phase diagrams of the first kind, thermodynamic quantities for a phase boundary reaction, 4Fe3O4+O2(g)=6Fe2O3, were extracted and compared with those for other systems of ferrite.