Spinel formation free energy

Few experimental data exist for the free-energy composition relationships in mixed-oxide or -sulphide solutions, for example (Fe,Cr)304 or (Fe,Cr)i cS i . so it is necessary to use various solution models to estimate these data. Furthermore, few data exist for the free energies of formation of compounds of fixed stoichiometry, such as the sulphide spinel FeCr2S4, and these must be estimated. 

In ceramics we are often dealing with multicomponent systems such as the ternary quartz, clay, and feldspar used in the formation of porcelains or the binary NiO, AI2O3 that react to form a spinel. Equilibrium, at constant T and P, is reached in these systems when the chemical potential, Pi, of a component is the same in all of the phases in which it is present. The chemical potential, or the partial molar free energy of a component in a given phase, is defined as... 

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