Phase relations in the systems Cu–O–R₂O₃(R = Tm, Lu) and Gibbs energies of formation of Cu₂R₂O₅ compounds

Abstract

The phase relations in the systems Cu–O–R₂O₅(R = Tm, Lu) have been determined at 1273 K by X-ray diffraction, optical microscopy and electron probe microanalysis of samples equilibrated in evacuated quartz ampules and in pure oxygen. Only ternary compounds of the type Cu₂R₂O₅ were found to be stable. The standard Gibbs energies of formation of the compounds have been measured using solid-state galvanic cells of the type, Pt|Cu₂O + Cu₂R₂O₅+ R₂O₅‖(Y₂O₃)ZrO₂‖CuO + Cu₂O‖Pt in the temperature range 950–1325 K. The standard Gibbs energy changes associated with the formation of Cu₂R₂O₅ compounds from their binary component oxides are: 2CuO(s)+ Tm₂O₃(s)→Cu₂Tm₂O₅(s), ΔG°=(10400 – 14.0 T/K)± 100 J mol^–1, 2CuO(s)+ Lu₂O₃(s)→Cu₂R₂O₅(s), ΔG°=(10210 – 14.4 T/K)± 100 J mol^–1 Since the formation is endothermic, the compounds become thermodynamically unstable with respect to component oxides at low temperatures, Cu₂Tm₂O₅ below 743 K and Cu₂Lu₂O₅ below 709 K. When the chemical potential of oxygen over the Cu₂R₂O₅ compounds is lowered, they decompose according to the reaction, 2Cu₂R₂O₅(s)→2R₂O₃(s)+ 2Cu₂O(s)+ O₂(g) The equilibrium oxygen potential corresponding to this reaction is obtained from the emf. Oxygen potential diagrams for the Cu–O–R₂O₃ systems at 1273 K are presented.