System Cu-Rh-O: Phase diagram and thermodynamic properties of ternary oxides CuRhO₂ and CuRh₂O₄

Abstract

An isothermal section of the phase diagram for the system Cu-Rh-O at 1273 K has been established by equilibration of samples representing eighteen different compositions, and phase identification after quenching by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive analysis of X-rays (EDX). In addition to the binary oxides Cu₂O, CuO, and Rh₂O₃, two ternary oxides CuRhO₂ and CuRh₂O₄ were identified. Both the ternary oxides were in equilibrium with metallic Rh. There was no evidence of the oxide Cu₂Rh₂O₅ reported in the literature. Solid alloys were found to be in equilibrium with Cu₂O. Based on the phase relations, two solid-state cells were designed to measure the Gibbs energies of formation of the two ternary oxides. Yttria-stabilized zirconia was used as the solid electrolyte, and an equimolar mixture of Rh+Rh₂O₃ as the reference electrode. The reference electrode was selected to generate a small electromotive force (emf), and thus minimize polarization of the three-phase electrode. When the driving force for oxygen transport through the solid electrolyte is small, electrochemical flux of oxygen from the high oxygen potential electrode to the low potential electrode is negligible. The measurements were conducted in the temperature range from 900 to 1300 K. The thermodynamic data can be represented by the following equations: 1/2Cu₂0+Rh₂0₃→CuRhO₂ A_f(ox)G°/J mol^-1 = - 18,040 + 0.975 T/K (+ 40) CuO + Rh₂0₃ →CuRh₂0₄ A_f(ox)G°/J mol^-1 = - 23,760 + 1-92 TIK (+ 325), where Δ_f(ox) G^o is the standard Gibbs energy of formation of the interoxide compounds from their component binary oxides. Based on the thermodynamic information, chemical potential diagrams for the system Cu-Rh-O were developed.