Tie-lines and mixing properties of solid solutions in the system CaO-SrO-PbO-O at 1100 K

Abstract

Phase relations in the system CaO-SrO-PbO-O₂ at 1100 K have been determined by equilibrating samples with different compositions in air, oxygen, or evacuated ampoules for 7 days and characterizing quenched specimens by optical and scanning electron microscopy, energy-dispersive analysis of x-rays (EDX), and x-ray diffraction (XRD). There is a solid-state miscibility gap in the pseudo-binary system CaO-SrO, and continuous solid solubility between Ca₂PbO₄ and Sr₂PbO₄ at 1100 K. Substitution of Ca for Sr occurs only to a limited extent (∼2 mol.%) in SrPbO3. The calcium-rich solid solutions (Ca_1−y Sr _y )₂PbO₄ characterized by y≤0.255 are in equilibrium with PbO in air; compositions with y≥0.255 coexist with (Ca_1−z Sr _z )PbO₃. There is a three-phase region involving the two monoxide solid solutions (Ca_1−x Sr _x)O on either side of the miscibility gap with x=0.24 and 0.71 and (Ca_1−y Sr _y )₂PbO₄ with y=0.96. Accurately determined are the locations of tie-lines between the solid solutions. Attainment of equilibrium was checked by the conventional tie-line rotation technique. The excess Gibbs energy of mixing of the solid solution with orthorhombic structure is obtained by an analysis of tie-line data; for the mixing of one mole of Ca and Sr represented by (Ca_1−y Sr _y )Pb_0.5O₂, ΔG^E =y(1−y) [15,840−2950 y] J/mol. The thermodynamic properties suggest the onset of immiscibility in this solid solution below 884 (±5) K. The miscibility gap is asymmetric with a critical composition at y=0.43 (±0.02). Inside the triangle (Ca_1−y Sr _y )Pb_0.5O₂−(Ca_1−z Sr _z )PbO₃−PbO, a small liquid-phase region is present close to the PbO corner, surrounded by three two-phase fields. Each corner of the approximately triangular liquid-phase region is associated with a three-phase field.