Phase relations, chemical potentials and thermodynamic properties of interoxide compounds in the system Ba-Pb-O

Abstract

Phase relations in the system Ba–Pb–O have been established by equilibrating 18 samples in the ternary in either pure oxygen, inert gas or evacuated capsules at 1100 K. Phase identification of quenched samples was performed using optical and electron microscopy, X-ray diffraction and energy dispersive analysis of X-rays. Three interoxide compounds, Ba₂PbO₄, Ba₄Pb₃O₁₀ and BaPbO₃ were found to be stable: tie-lines connect each of these compounds with PbO and oxygen gas. Lead is present in tetravalent state in all the three interoxide compounds. All alloys and intermetallics (Ba₂Pb, Ba₅Pb₃ and BaPb) are in equilibrium with BaO. The oxygen chemical potential corresponding to the three-phase fields, BaO+PbO+Ba₂PbO₄, PbO+Ba₂PbO₄Ba₄Pb₃O₁₀ and PbO+Ba₄Pb₃O₁₀+BaPbO₃ were measured using solid state cells incorporating (Y₂O₃)ZrO₂ as the solid electrolyte in the temperature range 750–1080 K. A special cell design, with a buffer electrode in addition to the normal reference and measuring electrodes, was used to avoid polarization of the three-phase electrode. The standard Gibbs free energies of formation of the three ternary oxides were derived from the emf of three solid oxide galvanic cells. The results were cross-checked by measuring the chemical potential of BaO in the three-phase field PbO+BaPbO₃+O₂ (26.2 Pa) and Gibbs energy of formation of BaPbO₃ using a cell based on BaF₂ as the electrolyte. The following equations summarize the results: 2BaO+PbO+1/2O₂→Ba₂PbO₄; ΔG°=-202 640+91.67 T (±400) J mol^-1; 4BaO+3PbO+3/2O₂→Ba₄Pb₃O₁₀; ΔG°=-556 835+283.12 T (±900) J mol^-1; BaO+PbO+1/2O₂→BaPbO₃; ΔG°=-165 970+87.45 T (±200) J mol^-1. The results obtained in this study differ significantly from data reported in the literature. The earlier investigators incorrectly assumed that BaO exists at unit activity under pure oxygen in the temperature range 700–875 K.