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

Abstract

An isothermal section of the phase diagram for the system Gd–Pd–O at 1223 K has been established by equilibration of 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 solution were found to be in equilibrium 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):Δ_f(ox)G°/J mol⁻¹=−25,030+0.33T (±140), Gd₂PdO₄(s):Δ_f(ox)G°/J mol⁻¹=−25,350+0.84T (±135), Gd₂Pd₂O₅(s):Δ_f(ox)G°/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.