Phase equilibria in the system CaO-CoO-SiO₂ and gibbs energies of formation of the quaternary oxides CaCoSi₂O₆, Ca₂CoSi₂O₇ and CaCoSiO₄

Abstract

Phase relations in the pseudoternary system CaO-CoO-Si0₂ have been established at 1323 K. Three quaternary oxides were found to be stable: CaCoSi₂0₆ with clinopyroxene (Cpx), Ca₂CoSi₂0₇ with melilite (Mel), and CaCoSi0₄ with olivine (01) structures. The Gibbs energies of formation of the quaternary oxides from their component binary oxides were measured using solid-state galvanic cells incorporating yttria-stabilized zirconia as the solid electrolyte in the temperature range of 1000-1324 K. The results can be summarized as follows: CoO (rs) + CaO (rs) + 2Si0₂ (Qtz)→CaCoSi₂0₆ (Cpx), ΔG°_f = -117920 + 11.26T (:t 150) J/mol CoO (rs) + 2CaO (rs) + 2Si02 (Qtz)→Ca₂CoSi₂0₇(Mel), ΔG°_f = -192690 + 2.38T(:t130) J/mol CoO (rs) + CaO (rs) + Si0₂ (Qtz)→CaCoSi0₄ (01), ΔG°_f = -100325 + 2.55T (:t 100) J/mol where rs = rock salt (NaCl) structure and Qtz = quartz. The uncertainty limits correspond to twice the standard error estimate. The experimentally observed miscibility gaps along the joins CaO-CoO and CaCoSi0₄-C0₂Si0₄ were used to calculate the excess free energies of mixing for the solid solutions Ca_XCo_1-XO and (Ca_yCo_1-y)CoSi0₄:ΔG^E = X(l - X)[31975X + 26736(1 - X)] J/mol and ΔG^E = 23100 (±250) Y(l - Y)J/mol. A T-X phase diagram for the binary CaO-CoO was computed from the thermodynamic information; the diagram agrees with information available in the literature. The computed miscibility gap along the CaCoSi0₄-C0₂Si0₄ join is associated with a critical temperature of 1389 (±15) K. Stability fields for the various solid solutions and the quaternary compounds are depicted on chemical-potential diagrams for Si0₂, CaO, and CoO at 1323 K.