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

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 dif ferent compositions, and phase identification after quenching by optical an d scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy d ispersive 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 evid ence of the oxide Cu2Rh2O5 reported in the literature. Solid alloys were fo und 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 th e two ternary oxides. Yttria-stabilized zirconia was used as the solid elec trolyte, and an equimolar mixture of Rh + Rh2O3 as the reference electrode. The reference electrode was selected to generate a small electromotive for ce (emf), and thus minimize polarization of the three-phase electrode. When the driving force for oxygen transport through the solid electrolyte is sm all, electrochemical flux of oxygen from the high oxygen potential electrod e to the low potential electrode is negligible. The measurements were condu cted in the temperature range from 900 to 1300 K. The thermodynamic data ca n be represented by the following equations: 1/2 Cu2O + 1/2 Rh2O3 --> CuRhO2 Delta(f(ox))G degrees/J mol(-1) = - 18,040 + 0.975 T/K (+/- 40) CuO + Rh2O3 --> CuRh2O4 Delta(f(ox))G degrees/J mol(-1) = - 23,760 + 1.92 T/K (+/- 325), where Delta(f(ox))G degrees is the standard Gibbs energy of formation of th e interoxide compounds from their component binary oxides. Based on the the rmodynamic information, chemical potential diagrams for the system Cu-Rh-O were developed.