A thermodynamic analysis was made of the pseudobinary phase diagram. The solid solutions in this system exhibit strong positive deviations from ideality, as evidenced by the large excess free energy of mixing, . An excellent fit of the experimental liquidus and solidus points was obtained with and . These values designate the liquid as nearly ideal and the solid as bordering on phase separation. The latter is consistent with the observation that composition homogeneity is extremely difficult to achieve in this system. The large deviation from ideality in the solid is attributed to lattice strain resulting from the large lattice mismatch between the two components.