System La-Pd-O: phase diagram and thermodynamic properties of ternary oxides

Abstract

An isothermal section of the phase diagram for the system La-Pd-O at 1200 K has been established by equilibration of samples representing seventeen 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 1200 K. Two ternary oxides La₄PdO₇ and La₂Pd₂O₅ were identified. Liquid alloys and the intermetallic compounds LaPd, La₃Pd₄ and LaPd₃, were found to be in equilibrium with La₂O₃. 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 (emf) measurements, conducted in the temperature range from 925 to 1400 K, indicated the presence of a third ternary oxide La₂PdO₄, stable below 1171(±5) 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 La₂O₃ and PdO can be represented by the following equations: La₄PdO₇: Δ_f(ox)G° [Jmol^-1]= -46550+0.56T(±220). La₄PdO₇: Δ_f(ox)G° [Jmol^-1]= -42000+2.19T(±160).La₄PdO₇: Δ_f(ox)G° [Jmol^-1]= -73990+1.33T(±310).Based on the thermodynamic information, chemical potential diagrams for the system La-Pd-O were developed.