High–temperature phase chemistry of the system Gd–Pd–O

Abstract

Anisothermal sectionof the phase diagram for the system Gd–Pd–O at 1223 K has been established by equilibrationof samples and phase identification after quenching by optical and scanning electron microscopy, X–ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides Gd4PdO7,Gd2PdO4 and Gd2Pd2O5 were identified. Liquid alloys, the four inter–metallic compounds and Pd–rich solid solutionwere found to be inequilibrium with Gd2O3.Based on the phase relations, four solid–state cells were designed to measure the Gibbs energies of formation of the three ternary oxides in the temperature range from 920 to 1320 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to cross check the data. An advanced version of the solid–state cell incorporating a buffer electrode with yttria–stabilized zirconia solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode was used for high–temperature thermodynamic measurements. The standard Gibbs energy of formation of the inter–oxide compounds from their component binary oxides can be represented by the following equations:Gd4PdO7(s) : Δf(ox)G0/J mol–1 = –25,030 + 0.33T (±140), Gd2PdO4(s) : Δf(ox)f(ox)G0/J mol–1 = –25,350 + 0.84T (±135), Gd2Pd2O5(s) : Δf(ox)f(ox)G0/J mol–1 = –48,700 + 0.38T (±270)Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Gd–Pd–O are developed.