Phase Diagram of a Nickel‐Zinc Ferrite of Composition Ni 0.685 Zn 0.177 Fe 2.138 O 4+γ

The oxygen content of Ni 0.685 Zn 0.177 Fe 2.138 O 4+γ was determined gravimetrically at atmospheric pressure in varying Po 2 , 3.5 × 10 −4 to 1.0 atm at 600° to 1450°C. The phase boundary associated with the precipitation of α‐Fe 2 O 3 was determined from the change in slope of γ vs T plots observed on heating. Metastability is particularly evident for curves observed on cooling. Isacompositional lines (0.002 < γ < 0.045) are shown on a plot of log PO 2 vs 1/T. An enthalpy of ‐21.6 kcal/mol is calculated for the oxidation of Fe 2+ .