The CaO-SrO-CuO-O₂ system: Phase equilibria and thermodynamic properties at 1123 K

Abstract

Phase relationships in the CaO-SrO-CuO system in pure oxygen at 1.01 × 10⁵ Pa pressure were determined by equilibrating different compositions at 1123 K for ∼120 h and analyzing the phases present in the quenched samples using X-ray diffraction (XRD), optical and scanning electron microscopy, and energy dispersive analysis of X-rays (EDAX). Four solid solution series were observed in the system. The Ca_wSr_1-wO monoxide solid solution with rock-salt structure was found to exhibit an asymmetric miscibility gap. The mixing properties of the monoxide system were deduced using a subregular solution model. For the (Ca_xSr_1-x)₂CuO₃ series, a complete solid solution range with orthorhombic space groupImmm was obtained. Calcium substituted for strontium up to 68 at.% in SrCuO_2+δ and 51.5 at.% in Sr₁₄Cu₂₄O_41-δ. The tie lines between the solid solutions were determined accurately. The activity-composition relations in (Ca_xSr_1-x)₂CuO₃, Ca_ySr_1-y,CuO_2+δ, and (Ca_zSr_1-z)₁₄Cu₂₄O_41-δ solid solutions were determined from experimental tie lines. Activities in the (Ca_xSr_1-x)₂CuO₃ and Ca_ySr_1-yCuO_2+δ series were close to the predictions of the Temkin model. The behavior of the (Ca_zSr_1-z)₁₄Cu₂₄O_41-δ solid solution was more complex, with the activity of SrCu_(24/14)O_(41-δ)/14 exhibiting both positive and negative deviations from ideality. Gibbs energy of formation of the CaCuO_2+δ metastable phase at 1123 K was deduced from an analysis of the phase diagram.