The thermodynamics and phase behaviour of Tb₂Pd₂O₅ and Er₂Pd₂O₅

Abstract

Isothermal sections of the phase diagrams for the systems Ln–Pd–O (with Ln=Tb or Er) have been established by equilibration of samples at T=1223K, and phase identification after quenching by optical and scanning electron microscopy (OM, SEM), energy dispersive spectroscopy (EDS), and X-ray powder diffraction (XRPD). Two oxide phases were stable along the binary Tb–O: Tb₂O_3+x, a phase of variable composition, and Tb₇O₁₂ at T=1223K. The oxide PdO was not stable at this temperature. Only one ternary oxide Tb₂Pd₂O₅ was identified in the Tb–Pd–O system. No ternary compound was found in the system Er–Pd–O at T=1223K. However, the compound Er₂Pd₂O₅ could be synthesized at T=1075K by the hydrothermal route. In both systems, the alloys and inter-metallic compounds were all found to be in equilibrium with the lanthanide sesquioxide Ln₂O₃ (where Ln is either Tb or Er). Two solid-state cells, each incorporating a buffer electrode, were designed to measure the Gibbs energy of formation of the ternary oxides, using yttria-stabilized zirconia as the solid electrolyte and pure oxygen gas as the reference electrode. Electromotive force measurements were conducted in the temperature range (900–1275) K for Tb–Pd–O system, and at temperatures from (900–1075) K for the system Er–Pd–O. The standard Gibbs energy of Δ_fG∘_m, formation of the inter-oxide compounds from their component binary oxides Ln₂O₃ and PdO Δ_fG∘_m, are represented by equations linear in temperature. Isothermal chemical potential diagrams for the systems Ln–Pd–O (with Ln=Tb or Er) are developed based on the thermodynamic information.