Limits of the Thermodynamic Stability of Cobalt—Iron—Manganese Mixed Oxides at 1200°C

The thermodynamic stability of spinel and rock‐salt structure phases has been investigated for the quasi‐ternary mixed oxide system Co‐Fe‐Mn‐O at 1200°C and at total pressures of the order of 1 atm (∼1.01 × 10 5 Pa) using thermogravimetric and electrical conductivity measurements. The results reveal the stability limits of the spinel and rock‐salt structure phases with varying cationic composition and oxygen partial pressure at 1200°C. The oxygen partial pressure was varied over 12 orders of magnitude, from log 10 a o2 = 0 to log 10 a o2 = ‐12, by using CO/CO 2 and N 2 /O 2 gas mixtures and monitored by an electrochemical, stabilized zirconia‐based EMF cell. The maximum cobalt mole fraction used in the investigation was about 0.33.