THERMODYNAMIC INVESTIGATIONS OF BI2O3-MO (M = CA, SR, AND BA) SYSTEMS USING GALVANIC CELLS

Galvanic cells with MF2 solid electrolytes (M = Ca, Sr, and Ba) were used to determine activity-composition diagrams in the Bi2O3-MO systems. Activities of MO were determined from the measured electromotive force (EMF) values while those of Bi2O3 were estimated by Gibbs-Duhem integration. Equilibrium phases were identified by x-ray diffraction. In the stability range of the rhombohedral phase (beta2), the activity of Bi2O3 exhibited a drastic decrease with increasing amount of MO in all three systems. Bi2O3-SrO system showed the widest rhombohedral phase field and the lowest activity of Bi2O3 (0.117) in the SrO-richest composition, (Bi2O3)0.57(SrO)0.43, at 630-degrees-C. Partial molar entropies of dissolution of MO were determined from the temperature dependence of EMF. On the Bi2O3-rich side, partial molar entropies of dissolution of MO were compared with a theoretical calculation of the configuration part of the partial molar entropy. A free energy vs. composition diagram was constructed for the CaO-Bi2O3 system.