System Sr-Pb-O: phase equilibria and thermodynamics using solid-state cells with buffer electrodes

Abstract

The isothermal section of the phase diagram for the system Sr-Pb-O at 1100 K is established by equilibrating 18 compositions in the ternary and analyzing quenched samples using optical and scanning electron microscopy, EDX and XRD. Two ternary oxides, Sr₂PbO₄ and SrPbO₃, containing lead in the tetravalent state, are found to be stable. They coexist with PbO. Alloys and intermetallics are in equilibrium with SrO. The high-temperature thermodynamic properties of the ternary oxides are measured for the first time with a novel apparatus. Yttria-stabilized zirconia is used as the solid electrolyte and pure diatomic oxygen gas as the reference electrode. The new feature is the buffer electrode, which is introduced between the reference and measuring electrodes to absorb the electrochemical flux of oxygen through the solid electrolyte. This prevents the polarization of the measuring electrode and ensures accurate data. Measurements are made in the temperature range 860-1090 K. The standard Gibbs energies of formation of Sr₂PbO₄ and SrPbO₃ from SrO, PbO, and O₂ are obtained directly from the reversible cell emfs. The results can be represented by the equations: Sr₂PbO₄, ΔG^o_f(ox)/ J mol^-1 = -168 650 + 97.87 T/K (±330), and SrPbO₃, ΔG^o_f(ox)/J mol^-1 = -141 925 + 94.30 T/K (±290).