System Bi-Sr-O: synergistic measurements of thermodynamic properties using oxide and fluoride solid electrolytes

Abstract

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 solutions (δ, β and γ). An isothermal section of the phase diagram is established at 1050 K by phase analysis of quenched samples. Three compounds, Bi₄Sr₆O₁₅, Bi₁₄Sr₂₄O₅₂ and Bi₂Sr₆O₁₁, contain Bi⁵⁺ ions. The stability of these phases is a function of oxygen partial pressure. 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 constant temperature gives information on oxygen content of the compounds present at the electrodes. The chemical potentials of Bi₂O₃ in two-phase fields of the pseudo-binary Bi₂O₃-SrO are measured using cells incorporating (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; the maximum difference in the value of ΔG±_f (Bi_2mSr_nO_p) / (m+n) is ±350 J/mol of component binary oxides. The results are discussed in light of the phase diagram and compared with calorimetric and chemical potential measurements reported in the literature. The combined use of emf data from cells incorporating fluoride and oxide electrolytes enhances the reliability of derived data.