Structure of the αγ interface

We study the structure and thickness of the interface between two phases, α and γ, when they are, or are nearly, in equilibrium with an intermediate third phase β. If β is not present in bulk, but if it would have been in equilibrium with α and γ had it been present, then the αγ interface consists of a film of β of a thickness that diverges logarithmically with the total sample height. Far from a critical point of any of the phase equilibria, that β film will typically be about 100 Å thick. If, as a bulk phase, β is just slightly less stable than α and γ, by an amount measured by a dimensionless distance δ in the composition space, then when the total sample height is infinite the thickness of the β film at the αγ interface diverges proportionally to ln(1/δ). The composition and composition-gradient profiles through the αγ interface for small δ are calculated and displayed graphically.