Gibbs Energy of Formation of Cu2Yb2O5and Thermodynamic Stability of Cu2R2O5(R= Tb–Lu)

Abstract

The electromotive force (emf) of the solid oxide electrochemical cell,[formula]has been measured between 960 and 1320K. Based on the measured emf of the above cell as a function of temperature, the Gibbs energy change for the reaction[formula]is obtained as[formula]The Gibbs energy change for the above reaction when combined with that for Cu2O + CuO equilibrium from the literature gives, for the reaction[formula]It can be seen that the formation of Cu2Yb2O5from the component oxides is endothermic. Since Cu2Yb2O5is an entropy stabilized compound and its formation is endothermic from component oxides, Cu2Yb2O5is thermodynamically unstable relative to its component oxides below 853K. Earlier we reported the Gibbs energy of formation of Cu2Dy2O5, Cu2Ho2O5, and Cu2Er2O5from CuO andR2O3(R= Dy, Ho, Er). Since Cu2Yb2O5is also a member of the Cu2R2O5family of compounds whose crystal structure belongs to a noncentrosymmetric space groupP21nb, an attempt has been made to correlate their thermodynamic properties such as enthalpy and entropy of formation with the ionic radius of the trivalent rare-earth cation pressent in octahedral coordination. Based on this correlation, ΔH, ΔS, and ΔGfor the formation of Cu2Tb2O5, Cu2Tm2O5, and Cu2Lu2O5from the component oxides have been estimated.