COMBINED USE OF OXIDE AND FLUORIDE SOLID ELECTROLYTES FOR THE MEASUREMENT OF GIBBS ENERGY OF FORMATION OF TERNARY OXIDES - SYSTEM BI-CA-O

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 ps eudo-binary line Bi2O3-CaO. Four ternary oxides, Bi2Ca2O5, Bi6Ca4O13, Bi2CaO4 and Bi14Ca5O26 and two phases of variable composition (delta a nd beta) are identified. The chemical potential of CaO in two-phase fi elds of the pseudo-binary Bi2O3-CaO is determined as a function of tem perature using solid state cells based on single crystal CaF2 as the e lectrolyte. The chemical potential of Bi2O3 in the two-phase fields is measured using cells incorporating (Y2O3)ZrO2 as the solid electrolyt e. The standard Gibbs free energy of formation of each ternary oxide f rom the binary oxides is calculated independently from the chemical po tentials of CaO and Bi2O3 in two-phase fields on either side of the co mpound composition. The independent assessments agree closely; the max imum difference in the value of Delta G(f)(0) (Bi-2m CanO3m+n)/(m + n) is 170 J/mol of the component binary oxides. The results are discusse d 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 o f ternary oxides from component binary oxides are given by the followi ng equations: Bi2Ca2O5: Delta G(f,ox)(0)(J/mol) = -43800 + 5.79T(+/- 9 00) Bi6Ca4O13: Delta G(f,ox)(0)(J/mol) = -109800 + 7.04T (+/- 2300) Bi 2CaO4: Delta G(f,ox)(0)(J/mol) = -31910 + 2.31T (+/- 650) Bi14Ca5O26: Delta G(f,ox)(0)(J/mol) = -184560 + 12.76T (+/- 4320) Bi1.4Ca0.3O2.4(b eta): Delta G(f,ox)(0)(J/mol) = -12290(+/- 300) at 900 K Bi1.56Ca0.22O 2.56(beta): Delta G(f,ox)(0)(J/mol) = -9890(+/- 180) at 900 K.