Crystallization of Glasses: A Theoretical Analysis of the Role of Fragility and Polymorphism in Crystals

Here we develop an elasticity‐based theory of crystallization in glasses that incorporates structural heterogeneity, fictive temperature, and polymorph‐mediated pathways. In a glass, structural degrees of freedom are effectively frozen, so that the fictive temperature T _f remains higher than the ambient temperature T , rendering the system intrinsically out of equilibrium. A central result is that the crystal–glass interfacial penalty is renormalized in fragile systems by soft, liquid‐like regions, leading to a subquadratic mismatch energy scaling as ΣR ^3/2 rather than the classical R ² form. Applying this framework to ethanol, we show that nucleation proceeds preferentially via a two‐step route through a plastic crystalline polymorph. The associated barriers are dramatically reduced: the glass‐to‐plastic step exhibits barriers of only  ∼5 k _B T , compared to  ∼10 ² k _B T for the direct glass‐to‐crystal transition. This large separation explains the dominance of the Ostwald pathway and the emergence of a pronounced time–temperature–transformation (TTT) nose. In contrast, silica retains the classical R ² scaling due to its rigid network, leading to very large barriers and suppressed bulk nucleation.