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

Abstract

Because of the discovery of superconducting compounds in the systems Ca-Sr-Bi-Cu-O and Ca-Ba-Tl-Cu-O, there is considerable interest in phase relations in these systems. 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 Ca[sub 2]CuO[sub 3] and Ca[sub 0.828]CuO[sub 1.93] were identified at 1073 K. However, only Ca[sub 2]CuO[sub 3] 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[sub 0.828]CuO[sub 1.93], which can also be represented as Ca[sub 15]Cu[sub 18]O[sub 35], has been identified in an earlier investigation as Ca[sub 0.828]CuO[sub 2]. 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 compoundmore » Ca[sub 0.828]CuO[sub 1.93] (Ca[sub 15]Cu[sub 18]O[sub 35]) 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. 18 refs., 11 figs., 3 tabs.« less