Phase Relations in the System Cu─Ho─O and Stability of Cu₂Ho₂O₅

Abstract

The phase relations in the system Cu─Ho─O have been determined at 1300 K using X-ray diffraction, optical microscopy, and electron microprobe analysis of samples equilibrated in evacuated quartz ampules and in pure oxygen. Only one ternary compound, Cu₂Ho₂O₅, was found to be stable. The Gibbs free energy of formation of this compound has been measurer using the solid-state cell Pt, Cu₂O + Cu₂Ho₂O₅+ Ho₂O₃/(Y₂O₃)ZrO₂/CuO + Cu₂O, Pt in the temperature range of 973 to 1350 K. For the formation of Cu₂Ho₂O₅ from its binary component oxides, 2CuO(s) + Ho₂O₃(s)→Cu₂Ho₂O₅(s) ΔG°= 11190 − 13.8T(±120)J·mol⁻¹ Since the formation is endothermic, Cu2Ho2O5 becomes thermodynamically unstable with respect to CuO and Ho₂O₃ below 810 K. When the oxygen partial pressure over Cu₂Ho₂O₅ is lowered, it decomposes according to the reaction 2Cu₂Ho₂O₅(s) →2Ho₂O₃(s) + 2Cu₂O(s) + O₂(g) for which the equilibrium oxygen potential is given by Δμo₂=−238510 + 160.2T(±450) J·mol⁻¹ The decomposition temperature at an oxygen partial pressure of 1.52 × 10⁴ Pa was measured using a combined DTA–TGA apparatus. Based on these results, an oxygen potential diagram for the system Cu─Ho─O at 1300 K is presented.