Phase relations and thermodynamic properties of condensed phases in the system calcium-copper-oxygen

Abstract

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 ternary 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 Ca_0.828CuO_1.93 were identified at 1073 K. However, only Ca2CuO3 was found to be stable at 1223 K. The thermodynamic properties of the two ternary compounds were determined using solid-state cells incorporating either an oxide or a fluoride solid electrolyte. The results for both types of cells were internally consistent. The compound Ca_0.828CuO_1.93, which can also be represented as Ca₁₅Cu₁₈O₃₅, has been identified in an earlier investigation as Ca_0.828CuO₂. Using a novel variation of the galvanic cell technique, in which the emf of a cell incorporating a fluoride electrolyte is measured as a function of the oxygen potential of the gas phase in equilibrium with the condensed phase electrodes, it has been confirmed that the compound Ca_0.828CuO_1.93 (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.