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

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