Equilibrium states between structure elements in solids

Equilibrium states between structure elements in solids 

For the chemical potential (and for all the partial molar values) of a structure element, we again find the same difficulty as encountered in the case of ions. Indeed, it is impossible to maintain the quantities of elements for all types of structure elements of a solid, except for one of them because, in addition, the electrical neutrality but also the ratios between sites must be preserved in a transformation. In order to circumvent the difficulty, we use the same approach as with the ions, 

as the expression of the affinity presents no problem, the deduction of the law of mass action to write the equilibrium conditions for a quasi-chemical reaction applies naturally. 

- Schottky adopted the reasoning of statistical thermodynamics to obtain the expression of the law of mass action in terms of fractions of sites. His idea was to use the model of the perfect solution with a mixture of structure elements within the same solid phase. 

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