The site fraction is defined as the fraction of sites on a sublattice occupied by a constituent. If a phase has more than one sublattice, then the constitution is given by site fractions for each constituent on every individual sublattice.
The site fraction and the mole fraction have the same numerical value if a phase has no sublattices.
In Thermo-Calc an aqueous solution phase is assumed to have one sublattice. The site fraction is thus equal to the mole fraction of the species in the aqueous phase.
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