Phase relations and thermodynamic properties of condensed phases in the system Sr-Cu-O

Abstract

The phase diagram of the system Sr-Cu-O has been determined at 1173 K. Several compositions in the ternary system were quenched after equilibration, and the phases present were identified by optical microscopy, X-ray diffraction, and electron proble microanalysis. Four ternary compounds, SrCu₂O₂, Sr₂CuO₃, SrCuO_2+δ, and Sr₁₄Cu₂₄O₄₁, have been identified. Values for the nonstoichiometric parameter d in SrCuO_2+δ corresponding to the different three-phase fields have been determined using the hydrogen reduction technique. The thermodynamic properties of all the ternary oxides were determined using solidstate cells incorporating either oxide or fluoride electrolyte. The results from both types of cells were internally consistent. A discrepancy in the recent literature regarding thermodynamic properties of Sr₂CuO₃ has been resolved. The compound Sr₁₄Cu₂₄O₄₁ has been identified in some of the earlier investigations as Sr₃Cu₅O_δ+z. 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 oxygen potential of the gas phase in equilibrium with the condensed phase electrode, the formula Sr₁₄Cu₂₄O₄₁ has been confirmed. The compound does not have significant nonstoichiometry. The phase relations in the system Sr-Cu-O have been evaluated as a function of temperature and oxygen partial pressure from the thermodynamic data.