System Pr-Pd-O : phase diagram and thermodynamic properties of ternary oxides using solid-state cells with special features

Abstract

An isothermal section of the phase diagram for the system Pr-Pd-O at 1223 K has been established by equilibration of samples representing 13 different compositions, and phase identification after quenching by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive analysis of X-rays (EDX). The binary oxide PdO was not stable at 1223 K. Three oxide phases were stable along the binary Pr-O; Pr₂O₃, Pr₇O₁₂ and a phase of variable composition between these two oxides designated as σ. Two ternary oxides Pr4PdO7 and Pr₂Pd₂O₅ were identified and their crystal structures were determined. Liquid alloy, the intermetallic compounds PrPd, Pr₃Pd₄, PrPd₃, PrPd₅, and Pd-rich solid solution (Pd_s.s.) were found to be in equilibrium with Pr₂O₃. Based on the phase relations, two solid-state cells were designed to measure the Gibbs energies of formation of the two ternary oxides. An advanced version of the solid-state cell incorporating a buffer electrode was used for high temperature thermodynamic measurements. The function of the buffer electrode, placed between reference and working electrodes, was to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. Electromotive force measurements, conducted in the temperature range 925-1400 K, indicated the presence of a third ternary oxide Pr₂PdO₄, stable below 1150 ± 9 K. Additional solid state cells were designed to study this compound. The standard Gibbs energy of formation of the interoxide compounds from their component binary oxides Pr₂O₃ and PdO can be represented by the following equations: Pr₄PdO₇: Δ_f(ox)G°/J.mol^-1 = -43 540 + 0.74 T (± 150) Pr₂PdO₄: Δ_f(ox)G°/J.mol^-1 = -38 805 + 1.91 T (± 120) Pr₂Pd₂O₅: Δ_f(ox)G°/J.mol^-1 = -68 640 + 1.28 T (± 270) Crystallographic data for all three ternary oxides have been determined based on the thermodynamic information, isobaric phase diagrams and isothermal chemical potential diagrams for the system Pr-Pd-O are developed.