Gibbs Phase Rule describes the nature of phase boundaries on phase diagrams, and is a foundational principle in materials thermodynamics. In Gibbs original derivation, he stipulates that the Phase Rule applies only to simple systems--defined to be homogeneous, isotropic, uncharged, and large enough that surface effects can be neglected; and not acted upon by electric, magnetic or gravitational fields. Modern functional materials; spanning nanomaterials, multiferrorics, materials for energy storage and conversion, colloidal crystals, etc.; are decidedly non-simple, leveraging various additional forms of thermodynamic work to achieve their functionality. Here, we extend Gibbs original arguments on phase coexistence to derive a generalized Phase Rule, based in the combinatorial geometry of high-dimensional convex polytopes. The generalized Phase Rule offers a conceptual and mathematical framework to interpret equilibrium and phase coexistence in advanced modern materials.
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