Phase equilibria in the cadmium-gallium-mercury system

The critical temperature of a cadmium—gallium mixture is increased from 295 °C to 328 °C by the addition of mercury. This maximum critical temperature is attained at a mole fraction mercury XHg = 0.2 and a mole fraction gallium xGa = 0.5. Isothermal sections through the iternary phase diagram between 295 and 328 °C contain an isolated miscibility gap with two critical points. The ternary liquid is described by an Ising model. A hierarchy of three succes sively better mathematical approximations for the entropy function: point, pair and tetrahedron approximation, is briefly delineated. Equations are given to express the composition and the temperature of the ternary critical point in the three quasi-chemical interaction coefficients. Comparing the calculated miscibility gap with the experimentally determined one shows that the Ising model predicts too high iternary critical temperatures. Taking into account 3- and 4-particle interaction, the binary cadmium-gallium binodale is described in a better way. However, the calculated ternary maximal critical temperature is still too high.