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 microprob e analysis of samples equilibrated in evacuated quartz ampules and in pure oxygen. Only one ternary compound, Cu2Ho2O5, was found to be stab le. The Gibbs free energy of formation of this compound has been measu red using the solid-state cell Pt,Cu2O + Cu2Ho2O5 + Ho2O3/(Y2O3)ZrO2/C uO + Cu2O,Pt in the temperature range of 973 to 1350 K. For the format ion of Cu2Ho2O5 from its binary component oxides, 2CuO(s) + Ho2O3(S) - -> Cu2Ho2O5(s) DELTAG-degrees = 11190 - 13.8T(+/- 120) J-mol-1 Since t he formation is endothermic, CU2Ho2O5 becomes thermodynamically unstab le with respect to CuO and Ho2O3 below 810 K. When the oxygen partial pressure over Cu2Ho2O5 is lowered, it decomposes according to the reac tion 2Cu2Ho2O5(s) --> 2Ho2O3(s) + 2Cu2O(S) + O2(g) for which the equil ibrium oxygen potential is given by DELTAmu(O2) = - 238510 + 160.2T(+/ - 450) J.mol-1 The decomposition temperature at an oxygen partial pres sure of 1.52 x 10(4) Pa was measured using a combined DTA-TGA apparatu s. Based on these results, an oxygen potential diagram for the system Cu-Ho-O at 1300 K is presented.