High temperature phase chemistry of system Eu-Pd-O

Abstract

An isothermal section of the phase diagram for the system Eu-Pd-O at 1223 K has been established by equilibration of samples representing 20 different compositions, and phase identification after quenching by optical and scanning electron microscopy, X-ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides, Eu₄PdO₇, Eu₂PdO₄, and Eu₂Pd₂O₅, were identified. Liquid alloys and the intermetallic compounds EuPd₂ and EuPd₃ were found to be in equilibrium with EuO. The compound EuPd₃ was also found to coexist separately with Eu₃O₄ and Eu₂O₃. The oxide phase in equilibrium with EuPd₅ and Pd rich solid solution was Eu₂O₃. 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 925 to 1350 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to crosscheck the data. An advanced version of the solid state cell incorporating a buffer electrode with yttria stabilised 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. Equations for the standard Gibbs energy of formation of the interoxide compounds from their component binary oxides Eu₂O₃ with C type structure and PdO have been established. Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Eu-Pd-O have been developed.