Relation of the Thermochemistry and Phase Diagrams of Condensed Systems

Abstract

The relation between the thermochemistry and the phase diagram of a condensed system is an identity. Although this identity was rather well known to the chemists, metallurgists, and ceramists who practiced in the early decades of this century, most practitioners educated after the second world war seem to be unaware of the equivalence. Indeed, the field of phase diagrams is almost exclusively tilled by metallurgists and ceramists, while thermochemical aspects of condensed systems is the exclusive province of chemists. This dichotomy is extremely wasteful since the integrated description of a system permits interrelation of phase diagram and thermochemical data yielding a more complete description of a given system. The description of the magnesium—gallium—aluminum system shown in Table 1 and Figure 1 illustrates the interrelation between thermochemical and phase diagram data and serves to review the whole problem for those who are accustomed to dealing only with the component parts. Table 1 summarizes the free energy of formation of Mg—Ga and Mg—Al compounds at 300°K. These free energies of formation have been calculated from measurements of the thermochemical properties of the Mg—Ga and Mg—Al systems^(1,2) using techniques which have been described in detail⁽³⁾ for combining phase diagram and thermochemical data.