Abstract
Galvanic cells with 
solid electrolytes 
were used to determine activity‐composition diagrams in the 
systems. Activities of 
were determined from the measured electromotive force (EMF) values while those of 
were estimated by Gibbs‐Duhem integration. Equilibrium phases were identified by x‐ray diffraction. In the stability range of the rhombohedral phase 
, the activity of 
exhibited a drastic decrease with increasing amount of
in all three systems. 
system showed the widest rhombohedral phase field and the lowest activity of 
(0.117) in the 
composition, 
, at 630°C. Partial molar entropies of dissolution of 
were determined from the temperature dependence of EMF. On the 
side, partial molar entropies of dissolution of 
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 
system.