PHASE-EQUILIBRIA AND THERMODYNAMIC PROPERTIES OF TERNARY OXIDES IN THE SYSTEM CO-MO-O

Coexisting phases in the ternary system Co-Mo-O at 1200 K have been id entified by X-ray diffraction, optical and scanning electron microscop y and energy dispersive analysis of X-rays. The samples were equilibra ted in evacuated quartz capsules and in oxygen gas. Two ternary oxides , Co2Mo3O8 and CoMoO4 were found to be stable. The compound Co2Mo3O8 w as found to be in equilibrium with Go-rich metallic solid solutions. O ther alloy phases with X-Mo>0.03 and intermetallics were found to be i n equilibrium with MoO2. The oxygen chemical potential corresponding t o the three-phase fields, CoO+Co2Mo3O8+CoMoO4, MoO2+Co2Mo3O8+CoMoO4 an d Co0.97Mo0.03 + MoO2+Co2Mo3O8 were measured using three separate soli d state cells incorporating (Y2O3)ZrO2 as the solid electrolyte and a mixture of Co+CoO as the reference electrode. The standard Gibbs energ ies of formation of the two interoxide compounds from binary oxides Co O and MoO2 (Delta G degrees(f,ox)) were deduced from the emf of the th ree cells. The high-temperature alpha-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 equa tions: 2CoO + 3MoO(2) --> Co2Mo3O8 Delta G degrees(f,ox) = - 30 510 9.36 T(+/-1100) J/mol(950 - 1280 K) CoO + MoO2 + 1/2O(2) --> COMoO4 De lta G degrees(f,ox+O2) = - 213 000 + 76.57 T(+/-470) J/mol(950 - 1145 K) = - 210 510 + 74.44 T(+/-420) J/mol(950 - 1145 K) Based on the info rmation from phase identification studies and thermodynamic measuremen ts, the isothermal section of the ternary phase diagram and oxygen pot ential diagram for the Co-Mo-O system at 1200 K have been developed. O 1998 Elsevier Science S.A. All rights reserved.