Activities and equilibrium partition coefficients of solute constituents in the melts of oxide materials with and without solid solution

Most oxide melts consist of several constituent species, including ionic species, and they have their own equilibrium partition coefficients. They are not necessarily unity at the congruent point, although phase diagrams based on the oxide components show the partition coefficient to be unity for the oxide components simply because their concentrations are equal between the liquid and the solid at the congruent point. However, if the material has very little solid solution such as Li₂B₄O₇ whose congruent point matches its stoichiometric point, all the partition coefficients of its constituents are unity at the congruent point. In contrast, a material with a distinct solid solution, whose congruent composition is different from its stoichiometric composition, such as LiNbO₃, almost always shows segregation of ionic species in the liquid boundary layer even in the congruent melt and the partitioned ionic species in the solid do not make the charge neutrality.