On choosing a reference surface for a two-sublattice model

Hillert’s choice of a reference surface for the (A,B)a(C,D)c solution has vital advantages: the corresponding contribution to the Gibbs energy of a phase is straightforward to calculate; the surface is geometrically simple; it is continuously differentiable; the method can easily be generalized for multicomponent phases with any number of sublattices regardless of whether a component resides in one sublattice only or may inhabit several …