PHASE-RELATIONS IN THE SYSTEMS CU-O-R2O3 (R=TM,LU) AND GIBBS ENERGIESOF FORMATION OF CU2R2O5 COMPOUNDS

The phase relations in the systems Cu-O-R2O3 (R=Tm, Lu) have been dete rmined at 1273 K by X-ray diffraction, optical microscopy and electron probe microanalysis of samples equilibrated in evacuated quartz ampul es and in pure oxygen. Only ternary compounds of the type Cu2R2O5 were found to be stable. The standard Gibbs energies of formation of the c ompounds have been measured using solid-state galvanic cells of the ty pe, Pt\Cu2O+CU2R2O5+R2O3 \\ (Y2O3)ZrO2 \\ CuO+Cu2O \ Pt in the tempera ture range 950-1325 K. The standard Gibbs energy changes associated wi th the formation of CU2R2O5 compounds from their binary component oxid es are: 2CuO(s)+Tm2O3(S)-->Cu2TM2O5(S), DELTAG-degrees = (10400-14.0 T /K) +/- 100 J mol-1 2CuO(s)+Lu2O3(S)-->Cu2Lu2O5(S), DELTAG-degrees = ( 10210-14.4 T/K) +/- 100 J mol-1 Since the formation is endothermic, th e compounds become thermodynamically unstable with respect to componen t oxides at low temperatures, Cu2TM2O5 below 743 K and Cu2LU2O5 below 709 K. When the chemical potential of oxygen over the CU2R2O5 compound s is lowered, they decompose according to the reaction, 2Cu2R2O5(S)--> 2R2O3(S) + 2Cu2O(S) + O2(g) The equilibrium oxygen potential correspon ding to this reaction is obtained from the emf. Oxygen potential diagr ams for the Cu-O-R2O3 systems at 1273 K are presented.