SYSTEM BI-SR-O - SYNERGISTIC MEASUREMENTS OF THERMODYNAMIC PROPERTIESUSING OXIDE AND FLUORIDE SOLID ELECTROLYTES

Phase equilibrium and electrochemical studies of the ternary system Bi -Sr-O indicate the presence of six ternary oxides (Bi2SrO4, Bi2Sr2O5, Bi2Sr3O6, Bi4Sr6O15, Bi14Sr24O52, and Bi2Sr6O11) and three solid solut ions (delta, beta, and gamma). An isothermal section of the phase diag ram is established at 1050 K by phase analysis of quenched samples. Th ree compounds, Bi4Sr6O15, Bi14Sr24O52, and Bi2Sr6O11, contain Bi5+ ion s. The stability of these phases is a function of oxygen partial press ure. The chemical potentials of SrO in two-phase fields are determined as a function of temperature using solid-state cells based on single crystal SrF2 as the electrolyte. Measurement of the emf of cells based on SrF2 as a function of oxygen partial pressure in the gas at consta nt temperature gives information on oxygen content of the compounds pr esent at the electrodes. The chemical potentials of Bi2O3 in two-phase fields of the pseudobinary Bi2O3-SrO are measured using cells incorpo rating (Y2O3)ZrO2 as the solid electrolyte. The standard free energies of formation of the ternary oxides are calculated independently using emfs of different cells. The independent assessments agree closely; t he maximum difference in the value of Delta G(f)degrees (Bi2mSrnOp)/(m + n) is +/- 350 J/mol of component binary oxides. The results are dis cussed in the light of the phase diagram and compared with calorimetri c and chemical potential measurements reported in the literature. The combined use of emf data from cells incorporating fluoride and oxide e lectrolytes enhances the reliability of derived data.