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:

$$\begin{gathered} {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}Cu_2 O + {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}Eu_2 O_3 (C - rare earth) \to CuEuO_2 \hfill \\ \Delta _{f,ox} G^ \circ /J mol^{ - 1} = - 570 - 0.463 T/K ( \pm 20) \hfill \\ CuO + Eu_{_2 } O_3 (C - rare earth) \to CuEu_2 O_4 \hfill \\ \Delta _{f,ox} G^ \circ /J mol^{ - 1} = - 3530 - 5.96 T/K ( \pm 110) \hfill \\ \end{gathered} $$

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.