Phase equilibria and thermodynamic properties of ternary oxides in the system Co–Mo–O

Abstract

Coexisting phases in the ternary system Co–Mo–O at 1200 K have been identified by X-ray diffraction, optical and scanning electron microscopy and energy dispersive analysis of X-rays. The samples were equilibrated in evacuated quartz capsules and in oxygen gas. Two ternary oxides, Co2Mo3O8 and CoMoO4 were found to be stable. The compound Co2Mo3O8 was found to be in equilibrium with Co-rich metallic solid solutions. Other alloy phases with XMo>0.03 and intermetallics were found to be in equilibrium with MoO2. The oxygen chemical potential corresponding to the three-phase fields, CoO+Co2Mo3O8+CoMoO4, MoO2+Co2Mo3O8+CoMoO4 and Co0.97Mo0.03+MoO2+Co2Mo3O8 were measured using three separate solid state cells incorporating (Y2O3)ZrO2 as the solid electrolyte and a mixture of Co+CoO as the reference electrode. The standard Gibbs energies of formation of the two interoxide compounds from binary oxides CoO and MoO2 (ΔG°f,ox) were deduced from the emf of the three cells. The high-temperature α-modification of CoMoO4 was found to disorder at 1145(±5) K. The measurements on all the three cells were internally consistent. The results can be represented by the equations:2CoO+3MoO2→Co2Mo3O8ΔG°f,ox=−30 510+9.36T(±1100)J/mol (950−1280K)CoO+MoO2+12O2→CoMoO4ΔG°f,ox+O2=−213 000+76.57T(±470)J/mol (950−1145K)=−210 510+74.44T(±420)J/mol (1145−1280K)Based on the information from phase identification studies and thermodynamic measurements, the isothermal section of the ternary phase diagram and oxygen potential diagram for the Co–Mo–O system at 1200 K have been developed.