PHASE-EQUILIBRIA IN THE CADMIUM-GALLIUM-MERCURY SYSTEM

The critical temperature of a cadmium-gallium mixture is increased fro m 295-degrees-C to 328-degrees-C by the addition of mercury. This maxi mum critical temperature is attained at a mole fraction mercury x(Hg) = 0.2 and a mole fraction gallium x(Ga) = 0.5. Isothermal sections thr ough the ternary phase diagram between 295 and 328-degrees-C contain a n isolated miscibility gap with two critical points. The ternary liqui d is described by an Ising model. A hierarchy of three successively be tter mathematical approximations for the entropy function: point, pair and tetrahedron approximation, is briefly delineated. Equations are g iven to express the composition and the temperature of the ternary cri tical point in the three quasi-chemical interaction coefficients. Comp aring the calculated miscibility gap with the experimentally determine d one shows that the Ising model predicts too high ternary critical te mperatures. Taking into account 3- and 4-particle interaction, the bin ary cadmium-gallium binodale is described in a better way. However, th e calculated ternary maximal critical temperature is still too high.