The liquidus line and Gibbs free energy of formation of a crystalline compoundA _m B _n (c)—The linear temperature approximation

Abstract

The liquidus line for a compoundA _m B _n (c) which has a narrow homogeneity range and whose Gibbs free energy of formation varies insignificantly with composition is investigated. It is shown that any fit of a liquidus line with the customarily used liquidus equation is incomplete unless it also considers another, equally fundamental, liquidus equation containing the enthalpy and entropy of formation ofA _m B _n (c). This is easily done in the special case when the relative partial molar enthalpies and excess entropies in the liquid phase are independent of temperature. Then two auxiliary relations are obtained whose satisfaction insures that the best fit obtained with either liquidus equation is the same. Thus, if in addition to the usual requirement that an adequate fit be obtained for a liquidus line, it is also required that the auxiliary relations be satisfied, a much more stringent test is imposed upon the liquid phase model used. Activity coefficient expressions, containing those of the ideal, strictly regular, and quasiregular models as special cases, are given and applied, along with those for the quasichemical model, to InSb. Only the generalized model is satisfactory.