Combined Use of oxide and fluoride solid electrolytes for the measurement of gibbs energy of formation of ternary oxides: system Bi-Ca-O

Abstract

Phase equilibrium studies of the ternary system Bi-Ca-O at 1000 K and ambient pressure indicate that all the ternary oxides lie along the pseudo-binary line Bi₂O₃-CaO. Four ternary oxides, Bi₂Ca₂O₅, Bi6Ca4O₁₃, Bi₂CaO₄ and Bi₁₄Ca₅O₂₆ and two phases of variable composition (δ and β) are identified. The chemical potential of CaO in two-phase fields of the pseudo-binary Bi₂O₃-CaO is determined as a function of temperature using solid state cells based on single crystal CaF₂ as the electrolyte. The chemical potential of Bi₂O₃ in the two-phase fields is measured using cells incorporating (Y₂O₃)ZrO₂ as the solid electrolyte. The standard Gibbs free energy of formation of each ternary oxide from the binary oxides is calculated independently from the chemical potentials of CaO and Bi₂O₃ in two-phase fields on either side of the compound composition. The independent assessments agree closely; the maximum difference in the value of ΔG_f° (Bi₂mCanO_3m+n)/(m+n) is 170 J/mol of the component binary oxides. The results are discussed in the light of the phase diagram and compared with calorimetric and free energy measurements reported in the literature. The combined use of emf data from cells incorporating fluoride and oxide electrolytes enhances the reliability of derived data. Free energies of formation of ternary oxides from component binary oxides are given by the following equations: Bi₂Ca₂O₅:ΔG°_f,ox(J/mol)=-43800+5.79T(±900) Bi₆Ca₄O₁₃:ΔG°_f,ox(J/mol)=-109800+7.04T(±2300) Bi₂CaO₄:ΔG°_f,ox(J/mol)=-31910+2.31T(±650) Bi₁₄Ca₅O₂₆:ΔG°_f,ox(J/mol)=-184560+12.76T(±4320) Bi_1.4Ca_0.3O_2.4 (β):ΔG°_f,ox(J/mol)=-12290(±300) at 900 K Bi_1.56Ca_0.22O_2.56 (β):ΔG°_f,ox(J/mol)=-9890(±180) at 900 K.