PHASE-RELATIONS AND THERMODYNAMIC PROPERTIES OF CONDENSED PHASES IN THE SYSTEM CA-CU-O

The isothermal sections of the phase diagram for the system Ca-Cu-O at 1073 and 1223 K have been determined. Several compositions in the ter nary system were quenched after equilibration, and the phases present were identified by optical microscopy, X-ray diffraction, and electron probe microanalysis. Two ternary compounds Ca2CuO3 and Ca0.828CuO1.93 were identified at 1073 K. However, only Ca2CuO3 was found to be stab le at 1223 K. The thermodynamic properties of the two ternary compound s were determined using solid-state cells incorporating either an oxid e or a fluoride solid electrolyte. The results for both types of cells were internally consistent. The compound Ca0.828CuO1.93, Which can al so be represented as Ca15Cu18O35, has been identified in an earlier in vestigation ag Ca0.828CuO2. Using a novel variation of the galvanic ce ll technique, in which the emf of a cell incorporating a fluoride elec trolyte is measured as a function of the oxygen potential of the gas p hase in equilibrium with the condensed phase electrodes, it has been c onfirmed that the compound Ca0.828CuO1.93 (Ca15Cu18O35) does not have significant oxygen nonstoichiometry. Phase relations have been deduced from the thermodynamic data as a function of the partial pressure of oxygen for the system Ca-Cu-O at 873, 1073, and 1223 K.