Phase relations in the system Cu-Eu-O and thermodynamic properties of CuEu₂O₄ and CuEuO₂

Abstract

Phase relations in the system Cu-Eu-O have been determined by equilibrating samples of different average composition at 1200 K and by phase analysis after quenching using optical microscopy (OM), x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray (EDX). The equilibration experiments were conducted in evacuated ampoules and under flowing inert gas and pure oxygen. The Cu-Eu alloys were found to be in equilibrium with EuO. The higher oxides of europium, Eu₃O₄ and Eu₂O₃, coexist with metallic copper. Two ternary oxides CuEu₂O₄ and CuEuO₂ were found to be stable. The ternary oxide CuEuO₂, with copper in the monovalent state, can coexist with Cu, Cu₂O, Eu₂O₃ and CuEu₂O₄ in different phase fields. The compound CuEu₂O₄ can be in equilibrium with Cu₂O, CuO, CuEuO₂, Eu₂O₃, and O₂ gas under different conditions at 1200 K. Thermodynamic properties of the ternary oxides were determined using three solid-state cells based on yttria-stabilized zirconia as the electrolyte in the temperature range from 875 to 1250 K. The cells essentially measure the oxygen chemical potential in the three-phase fields: Cu+Eu₂O₃+CuEuO₂, Cu₂O+CuEuO₂+CuEu₂O₄, and Eu₂O₃+CuEuO₂+CuEu₂O₄. The thermodynamic properties of the ternary oxides can be represented by the equations: 1/2Cu₂O+1/2Eu₂O₃(C−rareearth)→CuEuO₂ Δ_f,oxG∘/Jmol⁻¹=−570−0.463T/K(±20) CuO+Eu₂O₃(C−rareearth)→CuEu₂O₄ Δ_f,oxG∘/Jmol⁻¹=−3530−5.96T/K(±110) Thermogravimetric analysis (TGA) studies in Ar+O₂ mixtures confirmed the results from emf measurements. An oxygen potential diagram for the system Cu-Eu-O at 1200 K was evaluated from the results of this study and information available in the literature on the binary phases.